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 at 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.
Queen's 90th: What's happening and when?
21 April — The Queen will unveil a plaque on Windsor's Queen's Walkway, before lighting the first of a chain of beacons. Gun salutes will also be held across the UK
12-15 May — Celebrations featuring hundreds of horses and performers in the grounds of Windsor Castle
10 June — A national service of thanksgiving at St Paul's Cathedral
11 June — The annual Trooping the Colour at Horse Guards Parade
12 June — The Queen will host a street party for 10,000 people in the Mall
The monarch's birthday celebrations get under way later with a walkabout in Windsor.
Accompanied by the Duke of Edinburgh, she will unveil a plaque marking The Queen's Walkway — a 6.3km trail that links 63 significant points in Windsor.
The trail was designed to recognise the moment the monarch broke the record on 9 September 2015 held by her great-great-grandmother Queen Victoria by being on the throne for 63 years and seven months.
Royal gun salutes will be fired from each of the UK's capital cities and at other authorised stations across the UK.
Most of the salutes will be 21 guns in length — the standard royal gun salute — and will be held in locations including the Tower of London, Hillsborough Castle, Cardiff Castle, and Edinburgh Castle.
In the evening the Queen will be accompanied by Prince Philip, the Prince of Wales and Duchess of Cornwall, as she lights the first of more than 900 beacons across the UK and the world to mark her birthday.
Members of the Army cadet force will take beacons to the top of the highest peaks of England, Scotland, Wales and Northern Ireland.
Some of the beacons will be specially-built gas-fuelled structures, while others will be traditional bonfires or braziers on top of tall wooden posts.
The monarch will also be presented with a birthday cake at the Guildhall by the Great British Bake Off champion Nadiya Hussain, who has baked an orange drizzle cake with a butter cream and marmalade filling.
To coincide with the Queen's birthday, the largest exhibition of the Queen's clothes and accessories ever shown in Scotland will open at the Palace of Holyroodhouse later.
The display has been selected to cover the Queen's life and reign and includes evening and day wear.
On Friday, US President Barack Obama and First Lady Michelle Obama will be joining the Queen for lunch at Windsor Castle.
The Queen each year celebrates two birthdays, with her actual birthday on 21 April and her official birthday on a Saturday in June.
Celebrations of her official birthday this year take place from 10-12 June.
The Queen has been married for 68 years, longer than any other British monarch
Her coronation was the first to be televised and was watched by over 20 million people in the UK
There have been 12 UK prime ministers, seven Archbishops of Canterbury and seven Popes during her reign
She has sat for more than 130 official portrait paintings
During her reign, she has visited 117 countries, travelling more than a million miles
In 2012 she became the second British monarch to celebrate a diamond jubilee, and in 2015 became the UK's longest reigning monarch