The Queen has been “a rock of strength for our nation” and the Commonwealth, Prime Minister David Cameron has said in a tribute to mark her 90th birthday.
The Prince of Wales has recorded a special radio broadcast for the day, in which he reads an edited passage from William Shakespeare’s Henry VIII.
Celebrations get under way later as the Queen takes part in a walkabout in Windsor and lights a symbolic beacon.
A photograph of the monarch with young Royal Family members has been released.
The image, one of three taken by celebrity photographer Annie Leibovitz, shows the Queen surrounded by her five great-grandchildren and her two youngest grandchildren.
Gun salutes take place around the UK starting from noon, while the prime minister is expected to lead tributes to Britain’s longest serving monarch in the Commons.
The reading by Prince Charles, to be broadcast by the BBC later, is an extract from a speech by Archbishop Thomas Cranmer to King Henry VIII after the birth of the future Queen Elizabeth I.
Mr Cameron is expected to say: “Her Majesty The Queen has lived through some extraordinary times in our world.
“From the Second World War to the rations with which she bought the material for her wedding dress.
“From presenting the World Cup to England at Wembley in 1966 to man landing on the moon three years later.
“From the end of the Cold War to peace in Northern Ireland.
“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 other Leibovitz photographs show the monarch walking in the grounds of Windsor Castle with four of her dogs and sitting with her daughter, the Princess Royal.
The image of the Queen with the young royals was taken in the Green Drawing Room, part of Windsor Castle’s semi-State apartments just after Easter.
In it, the Queen – in the tradition of royal portraiture – holds her youngest great-grandchild Princess Charlotte, who is 11 months old, in her arms.
Also appearing in the image is two-year-old Prince George, Zara Phillips’s two-year-old daughter Mia Tindall, who holds the Queen’s famous black handbag, and Peter Phillips’s daughters Savannah, five, and three-year-old Isla.
The Queen is also joined by the two youngest of her eight grandchildren – the Earl and Countess of Wessex’s children – James, Viscount Severn, eight, and Lady Louise Windsor.