Actors including Dame Judi Dench and Vanessa Redgrave are throwing their weight behind a campaign to save the Victoria and Albert Theatre Museum.
A treasure trove of costumes, set designs and props are stored in the museum, which has failed to raise the £12 million needed for redevelopment and now faces closure.
It would mean the collection, which includes Laurence Olivier's outfit from Richard III and Noel Coward's make-up box, could be sold.
Dame Diana Rigg and RSC founder Sir Peter Hall are also among the those urging V&A trustees not to shut the only national museum for the performing arts, which attracted 180,000 visitors last year.
The museum opened nearly 20 years ago in a subterranean space in Covent Garden surrounded by the capital's theatres. In a letter to the Times, the actors write: "We are horrified that this important and world-renowned museum could be closed and its collection broken up.
"We fear this means that most of the exhibits would no longer be available to the public."
Other items on display include the personal correspondence of Sir Michael Redgrave, star of The Dambusters and Hitchcock's The Lady Vanishes.
Sir Michael's daughter Vanessa described the impending closure as " a catastrophe", adding: "It's vital that it does not get absorbed in some shape into the V&A in South Kensington, where it would lose is coherence."
