The Queen will today open the new base of the East Anglian Air Ambulance (EAAA) where her grandson, the Duke of Cambridge, works as a helicopter pilot.
The Queen, joined by the Duke of Edinburgh, will open the charity's operating base, the Egerton-Smith Centre, at Cambridge Airport.
William, who has been working with the air ambulance - a registered charity - for over a year, is on duty today.
During their visit the royal couple will tour the new facilities and will meet some of the charity's medics, support staff and pilots.
After viewing one of EAAA's two operational H145 helicopters, the Queen will unveil a plaque, formally opening the new facility.
Launched as a charity in 2000 the organisation's pilots fly an average of five missions per day from bases in Cambridge and Norwich - and cover a wider geographical area spanning Norfolk, Suffolk, Cambridgeshire, Essex, Bedfordshire and Hertfordshire.
Later in the day the Queen will travel to Buckingham Palace where she will hold her last private audience with David Cameron before he formally offers his resignation as Prime Minister during the meeting.
Sometime after Mr Cameron has left, Theresa May, the new Conservative Party leader, will hold her first audience with the Queen.
She will become the Queen's 13th prime minister when she accepts the monarch's offer to form a new government and following tradition will "kiss hands'' with the head of state - in reality shaking hands.