Sir Elton John has helped his musical of Billy Elliot celebrate seven years in the West End, a week after being treated for a respiratory infection.
"I shouldn't really be here because I'm recovering from a bout of pneumonia," the 65-year-old told reporters.
"But the doctor said I could come and I didn't want to miss this for anything."
The singer spent a day in hospital last week but said he was now "raring to go" ahead of Monday's Diamond Jubilee concert outside Buckingham Palace.
"I'm very honoured to be asked to play at it," he said. "It's a great cast of performers and it's going to be a great night."
Sir Elton's illness forced him to pull out of three performances of his current Las Vegas residency.
"I picked up an infection in Las Vegas and I was walking around with it for a week before it turned into pneumonia," he explained.
"I didn't realise I had it and it just knocked me for six. But just being here tonight is cheering me up."
Sir Paul McCartney, Sir Tom Jones and Dame Shirley Bassey are also scheduled to perform on Monday as part of The Queen's Diamond Jubilee celebrations.
Wednesday's gala was held to mark Billy Elliot having played 3,000 performances at London's Victoria Palace theatre.
Its director Stephen Daldry and writer Lee Hall joined an audience that included the double Olympic medallists James Cracknell and Dame Kelly Holmes.
"London's very much our home and we really wanted to celebrate the best of British as well as this extraordinary landmark in the history of Billy Elliot," said Daldry.
"Tonight is an emotional night for us. The show is about family and this is an extraordinary family to be part of."
Based on Daldry's 2000 film, the West End musical opened in 2005 and won three Olivier awards the following year.
A Broadway production closed earlier this year, having been seen by an estimated 1.65 million people.
Sir Elton wrote the music for the show, which tells of a young boy who finds an unexpected outlet in ballet during the 1984 miners' strike.
The role of Billy, created by Jamie Bell on screen, was played on Wednesday by Harris Beattie, a 13-year-old from Aberdeen.
Also in attendance on Wednesday were members of the British Ladies Olympic Hockey Team, whom Daldry said had come direct from training.
Daldry, currently engaged as executive producer of ceremonies at the 2012 Games, said he was hopeful that productions like Billy Elliot would prosper this summer.
Last year Andrew Lloyd Webber said London theatres would find it "very tough" to attract audiences and that the Olympics would cause "a bloodbath of a summer" for the West End.
"People have been worried about it and those worries are absolutely valid," said Daldry, 52. "But we're putting on extra matinees and they seem to be selling.
"No one knows is the answer. But we're doing okay and hopefully everyone else is too."
"I think it'll be okay," agreed Sir Elton. "There's going to be so many people in London and there's so many things they're going to want to do.
"London has the best theatre in the world and I'm really optimistic people will go and support it.
"They'll come here and they'll want to do the lot."
After the show, Sir Elton briefly discussed the musical version of George Orwell's Animal Farm he and Hall are currently planning.
"It's completely dark and completely different to this," he said. "Lee's finishing the lyrics right now but I haven't started on the music yet."
Published in 1945, Animal Farm tells of a farm whose animals - led by the pigs Snowball and Napoleon - rebel against their human keeper.
"It has lots of pigs in it, all singing," said Hall, who was quick to dispel comparisons with 2011's West End musical Betty Blue Eyes.
Based on the Alan Bennett film A Private Function, the short-lived show featured an animatronic pig with Kylie Minogue's voice.
Hall, however, lightheartedly insisted his latest venture would feature "real" pigs that were "in training as we speak".