The Prince of Wales took his companion Camilla Parker Bowles to a royal gala performance of the musical Mamma Mia!
The hit show, which is celebrating five years in the West End, is based around Abba pop songs.
After the performance, which raised £100,000 for the Prince's Trust, Charles confessed he "knew every word".
The prince met the show's creator, Abba's Bjorn Ulvaeus, and one of the group's other stars, Anni-Frid Prinzessin Reuss.
Charles reportedly danced the night away on his 50th birthday to hits from Abba at a party at Highgrove.
"
It's got what you call the feel-good-factor
"
Camilla Parker Bowles
Backstage, he told the cast: "I knew every single word. It certainly dates me."
Meanwhile Mrs Parker Bowles told them: "It's got what you call the feel-good-factor - you come away feeling good."
The prince officially reopened the Prince of Wales Theatre, which has undergone a 10-month £7.5m restoration by its owner Sir Cameron Mackintosh.
Mamma Mia! is currently playing in eight countries. It is estimated that 17m people worldwide have seen the show.
Since it premiered in the UK at London's Prince Edward Theatre more than 2,000 performances have been watched by over three million people, grossing in excess of £105m.
It includes Abba hits like Dancing Queen, Knowing Me Knowing You, Take A Chance On Me and Gimme Gimme Gimme.
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