The Duke of Edinburgh has been taken to a London hospital with a bladder infection and will miss the rest of the Diamond Jubilee celebrations.
Buckingham Palace said in a statement that Prince Philip, 90, has been taken to King Edward VII Hospital in London as a "precautionary measure" and would remain under observation for a few days.
Earlier on Sunday Prince Philip had accompanied the Queen on the royal barge on the 80-minute journey down the River Thames for the Diamond Jubilee pageant.
The prince had treatment to clear a blocked coronary artery in December.
The Queen continued with her Jubilee public engagements on Monday, joining some 12,000 people at a concert outside Buckingham Palace.
Performers included Robbie Williams, who opened the show, Jessie J, Annie Lennox, Sir Tom Jones, Madness, Stevie Wonder, Sir Cliff Richard and Paul McCartney.
The Queen, dressed in a black cloak, was helped to her seat in the royal box by Prince Charles an hour-and-a-half into the celebration, the BBC reports.
The concert ended with a firework display after which the Queen by pressing a diamond-shaped crystal, igniting a beacon marking her 60 years on the throne.
Other beacons had earlier been lit across the UK and the Commonwealth.