Prince is nearing the end of his 21-date residency in London

The singer appeared unannounced at the start of the encore to sing a cover of The Beatles' The Long and Winding Road, accompanied by Prince on guitar.

Afterwards, the two superstars embraced and Prince said to the crowd: "You know who it is - Sir Elton John!"

The collaboration clearly inspired the Minneapolis musician, who crammed a further seven songs into his encore.

Among them were his 1980s classics When Doves Cry and Sign O' The Times.

The BBC's John Hand, who was at the concert, said: "Sir Elton has performed onstage with many of the top stars of the past three decades, including Eminem and George Michael, but this was still a surprise - albeit one which was much appreciated by the crowd."

Second encore

Earlier in the show, Sir Elton had drawn cheers and applause when the venue's video screens pictured him in the audience.

The 60-year-old singer recently gave the UK premiere of his Red Piano show at the O2 on one of Prince's rare nights off.

Fans who waited around at Thursday night's concert were given a second treat - as Prince and his band returned to the stage for a second encore fifteen minutes after the house lights went up.

The 49-year-old then took to the stage at the O2's more intimate second venue for an aftershow performance that lasted more than two hours.

Playing into the small hours, Prince covered another Beatles track - Come Together - and Led Zeppelin's Whole Lotta Love.

The latter was a tribute to the hard rock band who, earlier this week, announced they were to reform for a one-off show at the venue in November.