Platinum Jubilee: Queen thanks nation as Jubilee weekend begins
The Queen has said she is "inspired" by the goodwill of the nation as celebrations for her Platinum Jubilee begin.
In a Jubilee message, she thanked the public for organising events and said "many happy memories" would be made.
Millions are gearing up for street parties to mark the monarch's 70 years on the throne over the four-day bank holiday weekend.
A new official photograph of the Queen at Windsor Castle was also released.
As well as community celebrations across the country, there are a series of official events, starting with Thursday's Trooping the Colour parade and ending on Sunday with a Jubilee Pageant through London.
The monarch said: "I continue to be inspired by the goodwill shown to me, and hope that the coming days will provide an opportunity to reflect on all that has been achieved during the last 70 years, as we look to the future with confidence and enthusiasm."
The portrait released along with her message was taken by Ranald Mackechnie in the Victoria Vestibule at Windsor Castle earlier this year.
It shows the Queen looking contented, sitting on a cushioned window seat at the castle which has been her main home for the last couple of years. The famous Round Tower is visible through the window.
Mr Mackechnie took two previous portraits of the Queen alongside the Prince of Wales, the Duke of Cambridge and Prince George to mark her 90th birthday in 2016.
The traditional Trooping the Colour parade to mark the Queen's official birthday involves more than 1,500 officers and soldiers and 350 horses from the Household Division, as well as an RAF flypast.
The 96-year-old monarch, who has mobility issues and has recently cancelled several public appearances, is likely to join members of the Royal Family on the balcony at Buckingham Palace to watch the display.
The Duke and Duchess of Sussex are watching the parade, although they will not appear on the balcony.
The couple, who now live in the US, have a prime vantage point from the Duke of Wellington's former office, overlooking Horse Guards Parade, where they can see Prince Charles inspect the guardsmen and officers and take their salute in his mother's place.
They travelled with their children Archie, three, and Lilibet, who turns one on Saturday. Lilibet is the Queen's family nickname.
The Queen's son Prince Andrew, whose royal titles were returned to the Crown amid a lawsuit in the US, is not present.
History is being made
The Platinum Jubilee celebrates something that has never happened before.
No British monarch has ever reigned for 70 years - it's a remarkable length of time on the throne.
To put it in perspective, the Queen's reign is already longer than the reigns of her four predecessors put together.
Her time as monarch has stretched over multiple eras, across 14 different UK prime ministers and 14 US presidents.
Older readers might be scratching their heads finding it hard to believe it's 45 years since the Silver Jubilee and the long, hot summer of 1977.
The nearest comparison in length of reign is Queen Victoria's 63 years - and in international terms, the Queen isn't far behind the 72 years of the Sun King of France, Louis XIV.
Reaching the age of 96 is also unprecedented for a British monarch. The Queen is one of about 138,000 people in the UK over the age of 95, the great majority of them women.
Apart from Elizabeth II, only two other monarchs have even lived into their eighties - Victoria and George III.
On Thursday evening, more than 3,000 beacons will be lit across the UK and the Commonwealth in tribute to the Queen, with the Tree of Trees beacon illuminated outside the palace.
The monarch is to lead the lighting of the principal Jubilee beacon in a special ceremony at Windsor Castle, Buckingham Palace said.
Also on Thursday the Prince of Wales and Duchess of Cornwall will guest star in an episode of TV soap EastEnders, on BBC One at 19:30 BST.
Prince Charles and Camilla join the residents of Albert Square at their jubilee street party.
At a service of thanksgiving at St Paul's Cathedral on Friday, there will be other accommodations made for the Queen's comfort, with no ceremonial journey to the event.
If the monarch does attend, she will use a different entrance rather than scaling the steps.
And it is not certain she will make the planned trip to Epsom for the Derby on Saturday.
On that evening, the BBC's Party at the Palace concert - set on three stages in front of Buckingham Palace - will see Diana Ross, George Ezra, Alicia Keys and Nile Rodgers entertain a live crowd of 22,000 people and a television audience of millions.
At the concert, heir to the throne Prince Charles and his son, the Duke of Cambridge, will pay tribute to the Queen who will be watching on television.
On Sunday there will be street parties, picnics and barbecues across the UK with more than 85,000 Big Jubilee Lunches planned.
The finale of the weekend will be the Jubilee Pageant which will make its way through the streets of the capital with a cast of 6,000 performers and close to 200 celebrities.
It will end with Ed Sheeran singing the national anthem with a choir made up of "national treasures" outside Buckingham Palace.
Ahead of the celebrations, the Queen took a short break at Balmoral Castle, in Aberdeenshire, but returned to Windsor on Tuesday.
Saturday is the first birthday of Harry and Meghan's daughter Lilibet - who is named after the Queen's family nickname.
The Queen's granddaughter Princess Eugenie has led tributes to her in an article in the Spectator, describing her "grannie" as "a woman who has transcended time and has been that constant rock for so many when the world can feel so fragile".
She said she would love her son August to "have her patience, her calmness and her kindness, while always being able to laugh at himself and keep a twinkle in his eye".
Not everyone will be celebrating the occasion. Anti-monarchy campaign group Republic said a recent poll had shown the British public "just aren't that bothered about royal events".
Speaking for the group, Graham Smith said: "The Jubilee is not a national celebration, it is a carefully staged event to promote the monarchy and the royal brand."
- FROM CRIME SCENES TO LABORATORIES: Watch the new series of Silent Witness on iPlayer
- A CENTURY OF BLOODSHED: What started the witch hunts?