The Queen will not be attending the Epsom Derby during the Platinum Jubilee weekend as she wants to pace herself during her milestone celebrations.
The 96-year-old will be represented at the racecourse by her daughter the Princess Royal instead, The Sunday Times reports.
The Derby - which will take place on Saturday, June 4 - would have been a personal highlight for the Queen.
The Queen is a passionate horse owner and breeder and was expected to be greeted on the course with a special guard of honour including up to 40 of her past and present jockeys.
However, the newspaper has said the head of state is now "increasingly unlikely" to head to Epsom.
Buckingham Palace declined to comment, saying the Queen's attendance at events would not be confirmed until closer to the time.
However, the Queen has vowed to make a double balcony appearance as she defies her ailing "mobility concerns" to celebrate her 70-year reign.
Her Majesty will delight the crowds by taking to the Buckingham Palace frontage twice as she kick-starts her Jubilee celebrations.
She is expected to delight the crowds at the start of the celebrations after Trooping the Colour on Thursday and after the Pageant parade finale on Sunday.
A Royal source said: "The Queen is determined to take part in the Jubilee celebrations.
"Trooping was brought forward by one week to be part of the Jubilee this year and the Queen very much wants to be part of it."
The nation's longest reigning monarch, who is on a pre-Jubilee break resting in Balmoral, Scotland, has been facing episodic mobility problems in recent months.
This led her to miss a number of major events this year, including the State Opening of Parliament.
The monarch is also hoping to be able attend the service of thanksgiving at St Paul's Cathedral on Friday, with her wider family including, it is expected, the Duke and Duchess of Sussex and the Duke of York.
It could be the first time Harry and Meghan have been seen alongside the Windsor clan since Megxit and since they accused an unnamed senior royal of racism during their controversial Oprah interview.
The monarch is expected to spend time at some stage with the Sussexes, who are bringing their children Archie and Lilibet over from the US.
Lili celebrates her first birthday on the Saturday, but it is not yet known when she will meet her great-grandmother in person for the first time.
At the BBC's Party At The Palace concert in front of Buckingham Palace on Saturday night, both the Prince of Wales and the Duke of Cambridge are said to be preparing to pay public tributes to the Queen during the star-studded event, which features acts including Diana Ross and Queen + Adam Lambert.
The Queen is not expected to be there, but will watch on television instead from Windsor Castle.
On Sunday, thousands of people will gather across the country as more than 85,000 Big Jubilee Lunches and street parties are staged in celebration of the Queen's record-breaking 70 year reign.
The Met Office is predicting sunny blue skies and dry weather on Saturday with temperatures hitting 20C.
But those picnicking and taking part in the spectacular Pageant parade through central London on Sunday could face more unsettled weather, with a cloudier, chillier day and some risk of showers.
In a personal message ahead of the celebrations, Prime Minister Boris Johnson has praised the monarch's leadership, wisdom and lifelong service.
"With the first Platinum Jubilee ever, no monarch has ever served the country so long and, more importantly, no monarch has ever served it so well," Mr Johnson said.
"Providing leadership and wisdom, this remarkable woman has dedicated her life to serving her people and to her beloved Commonwealth.
"She has led this country through good times and bad, inspiring people to serve others and their communities to create the pride and allegiance that unites us all."
