The Queen celebrates her 90th birthday today - a milestone that will be marked both privately with family and publicly by the nation.
Tributes will be paid to the monarch - born on April 21 1926 - who has become the country's first nonagenarian sovereign.
Well-wishers are expected to sing Happy Birthday to the Queen when she goes on a walkabout close to her Windsor Castle home later to acknowledge the good wishes of the public.
And in the evening the night sky over her Berkshire home will be lit up by a beacon, started by the Queen, the first in a chain of more than 1,000 that will spread across the country and the globe.
The Prince of Wales is understood to be making a speech in tribute to his mother at the night-time event.
David Cameron will lead the nation in honouring the monarch's life when MPs debate a "humble address" to mark the Queen's birthday, proposed by the Prime Minister.
He is expected to say: "Throughout it all, as the sands of culture shift and the tides of politics ebb and flow, Her Majesty has been steadfast - a rock of strength for our nation, for our Commonwealth and on many occasions for the whole world."
The milestone birthday has been marked by the release of an historic photograph of the Queen surrounded by her five great-grandchildren - including Prince George and Princess Charlotte - and her two youngest grandchildren.
The image is one of three taken by celebrity photographer Annie Leibovitz who has also captured the Queen informally with her only daughter the Princess Royal, and in the final image with four of her dogs, her corgis Willow and Holly and dorgis Vulcan and Candy.
One unusual tribute will see the Houses of Parliament lit up red, white and blue in honour of the Queen's milestone.
And traditional gun salutes will thunder across the capital when 41 volleys are fired by the King's Troop Royal Horse Artillery in London's Hyde Park, and 62 rounds by the Honourable Artillery Company close to the Tower of London - the extra 21 volleys for the citizens of the City of London to show their loyalty to the monarch.
Later today the Queen, accompanied by the Duke of Edinburgh, will unveil a plaque marking The Queen's Walkway at the foot of Castle Hill before going on a walkabout with Philip.
The walkway - a four-mile self-guided walking trail of Windsor by The Outdoor Trust - has been created in honour of the Queen becoming the country's longest reigning monarch.
After the Queen lights the beacon, heir to the throne Charles will host a lavish private dinner in the castle for his mother, attended by her family.
When the Queen became the nation's longest-reigning monarch last autumn - passing Queen Victoria's record - she remarked that living to a ripe old age can bring many anniversaries: ''Inevitably a long life can pass by many milestones. My own is no exception.''
And in her Christmas broadcast the Queen made a light-hearted remark about turning 90, hinting she was aware of the significance of the milestone: ''I am looking forward to a busy 2016, though I have been warned I may have Happy Birthday sung to me more than once or twice.''
During her 64-year reign the monarch has been served by 12 prime ministers from Sir Winston Churchill to David Cameron, while Barack Obama, who will drop for lunch on Friday, is the 12th US president to hold office over the same period.
Over the decades Britain has undergone major transformations from technological advances like computers and supersonic flight to developments in society and the political landscape.
Her personal life has brought her happiness with the Queen, who acceded to the throne at the age of 25, now surrounded by her children, grandchildren and great-grandchildren.
But she endured her ''annus horribilis'' in 1992, the year Charles separated from wife Diana, the Duke of York split from Sarah, and the Princess Royal divorced Captain Mark Phillips.
The woman who is seen by many as a stabilising force in national life was never expected to be Queen when she was born on April 21 1926 in a town house in London's Mayfair, the first child of the Duke of York, later King George VI.
Her destiny was changed with the abdication of her uncle, Edward VIII, for the woman he loved - American divorcee Wallis Simpson.
Famously the Queen dedicated her life to her future role as monarch on her 21st birthday - vowing to serve the Commonwealth: ''I declare before you all that my whole life, whether it be long or short, shall be devoted to your service and the service of our great imperial family to which we all belong.''