Sir Ian McKellen, Dame Judi Dench and Richmond’s own Samantha Bond are among actors who signed a petition over the Arts Council withdrawing Orange Tree Theatre funding.
The petition was organised by actor Andrew MacBean on behalf of actors’ union Equity, who said he was angry about the "cynical" cut.
The Richmond theatre was informed of the loss of funding - which amounts to £365,000 a year - on new artistic director Paul Miller’s first day in the job.
The petition, signed by actors, directors and agents, ended up with almost 1,200 signatures, including household names such as David Tennant and Benedict Cumberbatch.
The theatre’s founder Sam Walters departed last month after 43 years at the helm and the 172-capacity playhouse is London’s only permanent theatre-in-the-round.
Mr MacBean said: "As more people got to hear about it I realised just how profound industry feeling was over this cut.
"It took Sam Walters 43 years to build the Orange Tree to where it is today.
"This cynical and dramatic one hundred per cent cut is potentially hugely damaging to the theatre.
"For such a relatively small amount of money why is this theatre being put in such jeopardy? It is an unacceptable risk."
The letter refers to the Orange Tree as being "one of the biggest producers of plays" in London and highlights the recent appointment of Mr Miller.
However, Neil Darlison, director at Arts Council England, believes the council can still work with the theatre in exploring alternative funding options.
He said: "We are currently in positive discussions with Orange Tree Theatre about its future plans, along with other funding options available from the Arts Council.
"We are committed to working with it to help the organisation realise its ambitions."
The funding the theatre received amounted to 24 per cent of its annual income.
The next opportunity to apply for funding as a National Portfolio Organisation will be at the end of the next 2015-2018 cycle.
Artistic director Mr Miller said the theatre would have to make "under-the-hood changes" to its financial model when its funding is discontinued in April next year.