The Queen hopes "many happy memories" will be created this weekend as the nation begins to celebrate the monarch's 70-year reign.
In a special message released as millions prepare to gather in her honour during four days of tributes and street parties, the Queen said she is looking to the future with "confidence and enthusiasm".
She wrote: "Thank you to everyone who has been involved in convening communities, families, neighbours and friends to mark my Platinum Jubilee, in the United Kingdom and across the Commonwealth.
"I know that many happy memories will be created at these festive occasions."
She added: "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 seventy years, as we look to the future with confidence and enthusiasm."
To mark the historic milestone, an official Platinum Jubilee portrait of the monarch, 96, has also been unveiled.
In the new official portrait, the Queen looks contented and happy, photographed in her Windsor Castle home, sitting on a cushioned window seat, with the historic residence’s famous Round Tower visible in the distance.
With her hands resting on her lap and her hair curled in its familiar style, the Queen is dressed in a dusky dove blue Angela Kelly coat made of wool Charmelaine.
The shiny pearl and diamante scalloped beading, embroidered around the collar and cascading down front trim, is perhaps seemingly a nod to her historic Platinum anniversary.
The Queen is the first British monarch to celebrate 70 years on the throne, and the commemorations get under way in full on Thursday with the traditional Trooping the Colour parade and balcony appearance.
The image was taken by photographer Ranald Mackechnie in the Victoria Vestibule in the Queen’s private apartments at Windsor.
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.
He said of being commissioned to take the official Jubilee portrait: “It is an honour to be asked and a privilege to undertake.”
Also visible in the background is the statue of King Charles II which stands in the Quadrangle of the Castle.
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The seated Queen has been facing mobility problems for a number of months and has said she has not been able to move in the past.
Royals fans gathered in central London will be hoping to see the Queen on the balcony after the Trooping parade, as well as at the service of thanksgiving in St Paul’s Cathedral on Friday and possibly again on the balcony after the carnival Pageant finale on Sunday.
As well as wearing her trademark pearls, the Queen also has on her wedding and diamond engagement ring, as she usually does, and her gold watch.
The Platinum festivities are poignantly the first major Jubilee celebrations of the Queen’s reign without her late husband the Duke of Edinburgh, who died in April last year.