- Britain holds third day of Platinum Jubilee celebrations
- Queen skipped Epsom Derby due to 'episodic mobility problems'
- Princes to speak at concert outside Buckingham Palace
- Harry and Meghan's daughter, Lilibet, turns one
Queen Elizabeth's son and heir Prince Charles and her grandson Prince William will pay tribute to the record-breaking monarch at a pop concert at Buckingham Palace on the third day of nationwide celebrations for her 70 years on the throne.
The "Party at the Palace", which will feature the likes of singers Alicia Keys and Diana Ross, is the main Platinum Jubilee event on Saturday, along with the Epsom Derby horse race.
The 96-year-old monarch, a huge horse-racing fan and owner of many thoroughbreds, was herself forced to skip the 243rd running of the Derby because of "episodic mobility problems" that also forced her to miss a thanksgiving service in her honour on Friday.
Her daughter Princess Anne, who competed in the three-day equestrian event in the 1976 Olympics, stood in for her mother, who has rarely missed the race during her record-breaking reign and watched on television from her Windsor Castle home.
Later on Saturday, Charles and his eldest son William, fated to follow his father as king, will make speeches at the palace concert where performers from around the world will entertain some 22,000 people. The queen herself had not been expected to attend.
Also appearing will be rock band Queen + Adam Lambert and singer Rod Stewart, and it will feature a specially recorded performance by Elton John.
Queen guitarist Brian May, who played the national anthem from the roof of the palace at a concert for Elizabeth's golden jubilee in 2002, hinted at another memorable moment.
"Then there was a moment when I wondered ... after Buckingham Palace roof where can you go? Well ..., you will see," he said.
Another participant, veteran U.S. musician and record producer Nile Rodgers, said he had no idea what he would be playing.
"I look at it like this - whatever the queen asked me to do or her representatives or whatever, I will do that," he told BBC TV.
Four days of celebrations to mark the monarch's 70 years on the throne began with a military parade, a Royal Air Force flypast, and the lighting of beacons across Britain and the world, with tens of thousands of people joining the festivities.
During Friday's National Service of Thanksgiving at St Paul's Cathedral in London, Archbishop of York Stephen Cottrell used a horse racing analogy in his sermon to pay tribute to the queen.
"Your Majesty, we are sorry that you're not here with us this morning, but we are so glad that you are still in the saddle," he said. "And we are glad that there is still more to come. So thank you for staying the course."
A sideshow to the main celebrations has been Prince Harry and his American wife Meghan making their first public appearance together in Britain since quitting official duties to move to Los Angeles two years ago, during which time their relationship with other royals has become strained.
Notably on Saturday, the official Twitter accounts for the monarch, Charles and William all sent messages almost simultaneously to mark the first birthday of Lilibet, the couple's daughter who is named after the queen.
Elizabeth had not met her great-granddaughter prior to the trip, and Buckingham Palace has not commented on newspaper reports they had now finally been introduced.
Harry and Meghan have become divisive figures, with supporters regarding them as a breath of fresh air for the tradition-bound monarchy, while critics and many newspapers pour scorn on their behaviour and commercial activities such as striking a deal with global streaming service Netflix.
"So Far Apart", the Daily Mail newspaper said on its front page about the lack of any obvious interaction between Harry and elder brother William at Friday's thanksgiving service.