Phoebe Waller-Bridge, James McAvoy and Toby Jones have thrown their support behind a letter warning that the UK theatre industry stands “on the brink of ruin”.

The trio of actors have joined a group of dozens of performers, writers, directors, trade bodies and unions in signing a letter to the Government which says many venues face the risk of permanent closure.

The letter states: “Without Government investment, theatres will be forced to close and may never return.

“The threat of British theatre being destroyed by accident is as real as it is bleak.

“It would not only be a spiritual tragedy but an economic one.”

Research by the Society of London Theatre and UK Theatre suggests 70% of theatres will run out of money by the end of 2020.

Culture “will play a crucial role as we emerge from the crisis”, the letter states.

“Theatre is one of the UK’s most dazzling success stories.

“In all its forms, whether drama, musical theatre, opera or dance, British theatre is a world-class cultural and economic force, with productions filling venues from Broadway to Beijing.”

The warning has been backed by the Society of London Theatre and UK Theatre trade body and the Bectu and Equity trade unions.

Actors Reece Shearsmith, Sharon D Clarke and Wendell Pierce and playwright Tom Stoppard also signed the letter, which was addressed to Prime Minister Boris Johnson, Chancellor Rishi Sunak and Culture Secretary Oliver Dowden.

The letter calls for the Government to support the theatre workforce by continuing the job retention scheme and setting up a new support package for freelancers working in the sector.

It also calls for tax reliefs for theatres and the businesses who supply them, as well as support to help make venues “Covid-19 secure”, and additional relief and investment funds.

Speaking at the daily Downing Street press conference, Mr Dowden said it would be “exceptionally difficult” for theatres to reopen because of social distancing rules.

Mr Dowden added that they operated on “wafer-thin” profit margins that made it difficult for them to reopen without lots of people in the audience.

He said he was looking to see what further support could be offered to the industry, adding that there would need to be “greater flexibility” to overcome practical obstacles to the return of live performance.

“So that’s why over the next week we will be convening experts in a targeted way, bringing together our leading performers in theatres, choirs and orchestras with medical experts and advisers,” he said.

“And the idea is that they will work together in detail to develop that road map which is so badly needed to performing safely – with a particular focus on piloting innovative ideas that may permit live performances.”

On Wednesday it was revealed that musicals Les Miserables, Mary Poppins, Hamilton and The Phantom Of The Opera will not return to the West End this year.

Theatre owner Sir Cameron Mackintosh urged politicians to take action to help the sector, adding: “Despite the Government engaging with the desperate pleas from everyone in the theatre industry, so far there has been no tangible practical support beyond offers to go into debt, which I don’t want to do.

“Their inability to say when the impossible constraints of social distancing will be lifted makes it equally impossible for us to properly plan for whatever the new future is.”

A spokeswoman for the Department for Digital, Culture, Media and Sport said: “The performing arts industry is one of the UK’s greatest success stories and we are doing all we can to support the sector through the pandemic.

“We are providing unprecedented assistance including a year’s business rates holiday, Government loans, the Job Retention Scheme that hundreds of theatres have already received support from and the Arts Council have provided a £160 million emergency response package.

“We are committed to opening up venues as soon as it is safe to do so and are working directly with the sector on detailed advice and guidance.”