The Duchess of Cambridge has been given a gift every pregnant woman travelling on the London Underground probably needs - a "Baby on board" badge.

Kate still has a slim figure but, at five months pregnant, her baby bump is growing by the week.

kate middleton

The Duchess shows off her new badge

She was given the practical present as she joined the Queen and Duke of Edinburgh at Baker Street Tube station to celebrate the 150th anniversary of the transport network.

The engagement was the Queen's first public event in more than a week but she looked well and chatted to senior London Underground managers, train drivers and station staff during her visit.

The Queen, who has been suffering the symptoms of gastroenteritis, had been forced to miss a number of engagements since she was first declared ill by Buckingham Palace almost three weeks ago.

As the royal party stood on a platform, London Underground chief operating officer Howard Collins made the presentation of the white badge printed with the words "Baby on board" and featuring the famous London Underground roundel.

Watching guests laughed as Mr Collins handed over the gift and Kate held it against her teal-coloured Malene Birger coat and joked "I'll make sure I wear it at home".

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Mr Collins said afterwards: "Kate said 'Oh yes, I've seen this before'. She used to travel on the Tube so she probably saw them then.

"She asked me 'How do they work?' and I said they're fantastic, they really do make a difference - saves men the embarrassment of having to guess if a woman is pregnant.

"She asked so many great questions of the staff and seemed really interested in our work."

The badges, available from Transport for London (TfL), overcome the awkwardness often felt by pregnant women of having to ask someone to give up their seat.

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  • 150 years of London Underground

    The Duchess of Cambridge during a visit to Baker Street Tube Station in London to mark 150th anniversary of the London Underground.

  • 150 years of London Underground

    The Duchess of Cambridge during a visit to Baker Street Tube Station in London to mark 150th anniversary of the London Underground.

  • 150 years of London Underground

    The Duchess of Cambridge during a visit to Baker Street Tube Station in London to mark 150th anniversary of the London Underground.

  • 150 years of London Underground

    The Duchess of Cambridge during a visit to Baker Street Tube Station in London to mark 150th anniversary of the London Underground.

  • 150 years of London Underground

    The Duchess of Cambridge during a visit to Baker Street Tube Station in London to mark 150th anniversary of the London Underground.

  • 150 years of London Underground

    The Duchess of Cambridge during a visit to Baker Street Tube Station in London to mark 150th anniversary of the London Underground.

  • 150 years of London Underground

    The Duchess of Cambridge during a visit to Baker Street Tube Station in London to mark 150th anniversary of the London Underground.

  • 150 years of London Underground

    The Duchess of Cambridge during a visit to Baker Street Tube Station in London to mark 150th anniversary of the London Underground.

  • 150 years of London Underground

    The Duchess of Cambridge and The Duke of Edinburgh, during a visit to Baker Street Tube Station in London to mark 150th anniversary of the London Underground.

  • 150 years of London Underground

    The Duchess of Cambridge during a visit to Baker Street Tube Station in London to mark 150th anniversary of the London Underground.

  • 150 years of London Underground

    The Duchess of Cambridge and The Duke of Edinburgh, during a visit to Baker Street Tube Station in London to mark 150th anniversary of the London Underground.

  • 150 years of London Underground

    The Duchess of Cambridge during a visit to Baker Street Tube Station in London to mark 150th anniversary of the London Underground.

  • 150 years of London Underground

    The Duchess of Cambridge, holds a badge that reads Baby on Board, given to her during a visit to Baker Street Tube Station in London to mark 150th anniversary of the London Underground.

  • 150 years of London Underground

    ALTERNATE CROPThe Duchess of Cambridge, holds a badge that reads Baby on Board, given to her during a visit to Baker Street Tube Station in London to mark 150th anniversary of the London Underground.

  • Duchess of Cambridge

    Britain's Kate, Duchess of Cambridge, accompanies Queen Elizabeth II and Prince Philip, not pictured, arrives at Baker Street underground station in London for a visit to mark the 150th anniversary of the London Underground, Wednesday, March 20, 2013. (AP Photo/Sang Tan)

  • 150 years of London Underground

    The Duchess of Cambridge, arrives by separate car, to join Queen Elizabeth II and the Duke of Edinburgh, at Baker Street Underground station in London for an event celebrating the 150th anniversary of the Tube.

  • Duchess of Cambridge

    Britain's Kate, Duchess of Cambridge, accompanies Queen Elizabeth II and Prince Philip, not pictured, leaves Baker Street underground station in London after a visit to mark the 150th anniversary of the London Underground, Wednesday, March 20, 2013. (AP Photo/Sang Tan)

  • 150 years of London Underground

    The Duchess of Cambridge, arrives by separate car, to join Queen Elizabeth II and the Duke of Edinburgh, at Baker Street Underground station in London for an event celebrating the 150th anniversary of the Tube.

  • 150 years of London Underground

    Queen Elizabeth II arrives with Duke of Edinburgh and the Duchess of Cambridge at Baker Street Underground station in London for an event celebrating the 150th anniversary of the Tube.

  • 150 years of London Underground

    The Duchess of Cambridge, arrives by separate car, to join Queen Elizabeth II and the Duke of Edinburgh, at Baker Street Underground station in London for an event celebrating the 150th anniversary of the Tube.

  • 150 years of London Underground

    The Duchess of Cambridge, arrives by separate car, to join Queen Elizabeth II and the Duke of Edinburgh, at Baker Street Underground station in London for an event celebrating the 150th anniversary of the Tube.

  • Duchess of Cambridge

    Britain's Kate, Duchess of Cambridge, right, accompanies Queen Elizabeth II, unseen, and Prince Philip, left, at Baker Street underground station in London for a visit to mark the 150th anniversary of the London Underground, Wednesday, March 20, 2013. (AP Photo/Sang Tan)

  • Duchess of Cambridge

    Britain's Kate, Duchess of Cambridge, accompanies Queen Elizabeth II and Prince Philip, not pictured, arrives at Baker Street underground station in London for a visit to mark the 150th anniversary of the London Underground, Wednesday, March 20, 2013. (AP Photo/Sang Tan)

  • Duchess of Cambridge, Queen Elizabeth II, Prince Philip

    Britain's Kate, Duchess of Cambridge, right, accompanies Queen Elizabeth II, left, and Prince Philip, center, as they arrive at Baker Street underground station in London for a visit to mark the 150th anniversary of the London Underground, Wednesday, March 20, 2013. (AP Photo/Sang Tan)

  • Duchess of Cambridge

    Britain's Kate, Duchess of Cambridge, right, accompanies Queen Elizabeth II and Prince Philip, third left and fourth, arrives at Baker Street underground station in London for a visit to mark the 150th anniversary of the London Underground, Wednesday, March 20, 2013. (AP Photo/Sang Tan)

  • Duchess of Cambridge

    Britain's Kate, Duchess of Cambridge, right, accompanies Queen Elizabeth II and Prince Philip, obscured at left, as they arrive at Baker Street underground station in London for a visit to mark the 150th anniversary of the London Underground, Wednesday, March 20, 2013. (AP Photo/Sang Tan)

  • 150 years of London Underground

    The Duchess of Cambridge, arrives by separate car, to join Queen Elizabeth II and the Duke of Edinburgh, at Baker Street Underground station in London for an event celebrating the 150th anniversary of the Tube.

  • Duchess of Cambridge

    Britain's Kate, Duchess of Cambridge, accompanies Queen Elizabeth II and Prince Philip, not pictured, arrives at Baker Street underground station in London for a visit to mark the 150th anniversary of the London Underground, Wednesday, March 20, 2013. (AP Photo/Sang Tan)

  • The Queen, Duke Of Edinburgh & Duchess Of Cambridge Visit Baker Street Underground Station

    LONDON, ENGLAND - MARCH 20: Catherine, Duchess of Cambridge makes an official visit to Baker Street Underground Station on March 20, 2013 in London, England. (Photo by Danny E. Martindale/Getty Images)

  • The Queen, Duke Of Edinburgh & Duchess Of Cambridge Visit Baker Street Underground Station

    LONDON, ENGLAND - MARCH 20: Catherine, Duchess of Cambridge makes an official visit to Baker Street Underground Station on March 20, 2013 in London, England. (Photo by Danny E. Martindale/Getty Images)

  • 150 years of London Underground

    A Baby on Board badge is shown as the Duchess of Cambridge, joined Queen Elizabeth II and the Duke of Edinburgh, at Baker Street Underground station in London for an event celebrating the 150th anniversary of the Tube.

  • Britain's Queen Elizabeth II steps off a parked train she was shown around at Baker Street underground station in London, for a visit to mark the 150th anniversary of the London Underground, Wednesday, March 20, 2013. The Queen made her first public engagement in more than a week Wednesday following her hospitalization for a stomach bug. The British head of state joined her husband Prince Philip and their granddaughter-in-law, Kate Duchess of Cambridge, for the event marking the 150th anniversary of London's sprawling subway system, affectionately known as the Tube. (AP Photo/Matt Dunham)

  • 150 years of London Underground

    ALTERNATE CROPThe Duchess of Cambridge, holds a badge that reads Baby on Board, given to her during a visit to Baker Street Tube Station in London to mark 150th anniversary of the London Underground.

  • 150 years of London Underground

    The Duchess of Cambridge arrives at Baker Street Underground Station to Mark 150th anniversary of the London Underground.

  • Duchess of Cambridge

    Britain's Kate, Duchess of Cambridge, accompanies Queen Elizabeth II and Prince Philip, not pictured, arrives at Baker Street underground station in London for a visit to mark the 150th anniversary of the London Underground, Wednesday, March 20, 2013. (AP Photo/Sang Tan)

  • 150 years of London Underground

    ALTERNATE CROPThe Duchess of Cambridge arrives at Baker Street Underground Station to Mark 150th anniversary of the London Underground.

  • 150 years of London Underground

    The Duke of Edinburgh and The Duchess of Cambridge arrive at Baker Street Underground Station to mark the 150th anniversary of the London Underground.

  • 150 years of London Underground

    The Duchess of Cambridge arrives at Baker Street Underground Station to mark the 150th anniversary of the London Underground.

  • 150 years of London Underground

    The Duchess of Cambridge arrives at Baker Street Underground Station to Mark 150th anniversary of the London Underground.

  • Duchess of Cambridge

    Britain's Kate, Duchess of Cambridge, accompanies Queen Elizabeth II and Prince Philip, not pictured, leaves Baker Street underground station in London after a visit to mark the 150th anniversary of the London Underground, Wednesday, March 20, 2013. (AP Photo/Sang Tan)

  • 150 years of London Underground

    The Duchess of Cambridge arrives at Baker Street Underground Station to mark the 150th anniversary of the London Underground.

  • 150 years of London Underground

    The Duchess of Cambridge, holds a badge that reads Baby on Board, given to her during a visit to Baker Street Tube Station in London to mark 150th anniversary of the London Underground.

  • 150 years of London Underground

    The Duchess of Cambridge arrives at Baker Street Underground Station to Mark 150th anniversary of the London Underground.

  • 150 years of London Underground

    The Duchess of Cambridge arrives at Baker Street Underground Station to Mark 150th anniversary of the London Underground.

  • 150 years of London Underground

    Queen Elizabeth II, during a visit to Baker Street Tube Station in London to mark 150th anniversary of the London Underground.

  • 150 years of London Underground

    Queen Elizabeth II, during a visit to Baker Street Tube Station in London to mark 150th anniversary of the London Underground.

  • 150 years of London Underground

    Queen Elizabeth II, during a visit to Baker Street Tube Station in London to mark 150th anniversary of the London Underground.

  • 150 years of London Underground

    Queen Elizabeth II, during a visit to Baker Street Tube Station in London to mark 150th anniversary of the London Underground.

  • 150 years of London Underground

    Queen Elizabeth II, during a visit to Baker Street Tube Station in London to mark 150th anniversary of the London Underground.

  • 150 years of London Underground

    Queen Elizabeth II, during a visit to Baker Street Tube Station in London to mark 150th anniversary of the London Underground.

  • 150 years of London Underground

    Queen Elizabeth II, during a visit to Baker Street Tube Station in London to mark 150th anniversary of the London Underground.