The world's longest-running play has been celebrating its golden anniversary in London's West End with a royal gala performance.

Murder mystery The Mousetrap, written by Agatha Christie, was attended by The Queen, who is marking her own 50 years on the throne, and the Duke of Edinburgh.

The performance at St Martin's Theatre in the West End reunited three of the original cast from 1952, including Sir Richard Attenborough and his wife Sheila Sim.

Monday evening's performance marks the play's 20,807th to date.

The Queen has joined the 10 million to see the play

Christie's famous whodunnit is the first stage production to achieve the milestone of half a century, having opened on 25 November, 1952.

More than 10 million people have seen the classic since it opened and the play has been performed in more than 40 countries and been translated into over 20 languages.

"It is like a London institution - like the ravens in The Tower of London," said Sir Richard Attenborough.

Christie's great-great grandsons, Joshua and Max Clementson presented the Queen with a posy and a giant mousetrap.

The royal couple met the current cast of the show following the performance.

The Mousetrap started out as a 30-minute play called Three Blind Mice, written by Christie for Queen Mary's 80th birthday in 1947, before it was made a full-length show.

Author bemused

In 1958 The Mousetrap became the longest running British show before breaking the world record in the mid-1970s.

The Queen received an ornamental mousetrap

Christie herself seemed a bit bemused by its appeal.

"It's not really frightening. It's not really horrible. It's not really a farce. But it has a little of all these things and perhaps that satisfies a lot of people," she once said.

The show transferred to its current location at St Martin's in 1974.

Almost 300 actors have performed the play with David Raven making the Guinness Book of Records as "most durable actor" for 4,575 performances as Major Metcalf.

Nancy Seabrooke was understudy for the role of Mrs Boyle for 6,240 performances, appearing just 72 times.

In 2000, the set was replaced for the first time since it opened, but the same design was kept.

Only a leather chair and the mantle clock have appeared in every show since 1952.

Actor Sir Antony Sher has praised the Prince of Wales and Camilla Parker Bowles for helping keep Shakespeare alive in British theatres.

He said the couple's patronage of Shakespeare's plays was boosting their popularity.

Sir Antony said: "His support for the Royal Shakespeare Company is phenomenal because he's a real fan. It doesn't seem to be a chore."