Dame Judi Dench is among a list of actors who have called for the Victoria and Albert's (V&A) historical Theatre Museum to be saved from closure.
Dame Judi Dench was among 17 other signatories
Dame Diana Rigg, Vanessa Redgrave and Peter Bowles also signed the letter which appeared in The Times newspaper.
It said they were "horrified" that the museum could close and its valuable collection be broken up and "would be no longer available to the public".
V&A trustees are expected to decide the museum's fate later this week.
The actors' letter added they thought it was "astonishing" that the Theatre Museum, in London's Covent Garden, could be closed "without adequate consultation with either museum users or theatre practitioners".
They called for "adequate funding" to develop the museum's current site and "make even more of its outstanding collections available".
The actors said it should remain near London's West End and the heart of theatreland, saying it would be lost if it was moved to the V&A's main site in South Kensington.
Vanessa Redgrave: Family items in the museum
Donald Sinden, Natasha Richardson, Miriam Margolyes and Jemma, Corin and Lynn Redgrave were among the other signatories to the letter.
Actors' union Equity has spearheaded the campaign to save the Theatre Museum.
Opened 20 years ago, the museum contains many documents and photographs from the Redgrave family, including the late Sir Michael Redgrave's diaries.
Costumes, books and archive footage from drama, dance, opera musical theatre and circus are among its other items.
Other treasures include Sir Laurence Olivier's costume for Richard III and Noel Coward's make-up box.
The V&A announced plans for a £12m redevelopment of the Theatre Museum two years ago, but struggled to raise sufficient funds.