When the playwright Simon Stephens complained that the recession had made theatre audiences more conservative, it could have been dismissed as sour grapes. The Olivier award-winner's latest play, The Trial of Ubu, had opened to mixed reviews and proved a commercial flop, playing on one snowy Saturday night to just 54 people in a 277-seat auditorium at London's Hampstead theatre.

Now, however, two of the UK's leading playwrights, Sir David Hare and Mark Ravenhill, have expressed disquiet that subsidised theatres are avoiding challenging or experimental work in favour of more familiar or feelgood fare.

Hare said many venues that once housed less commercial work were becoming increasingly mainstream, because of a desire to make up the shortfall caused by cuts to their funding.

"What's happening in the theatre is what's happening in the rest of society – there's a club class and an economy class," he said. "You have extremely prosperous boutique theatres, which are in public/private partnership in inner London, which are very successful at raising sponsorship. Then there's economy-class theatre, which is finding it much tougher, and that's mainly in the regions where people are not able to get that level of subsidy."

Comedies currently dominate London's theatre scene. On Tuesday, Josie Rourke kicked off her tenure as creative director of the Donmar with the Restoration comedy The Recruiting Officer. The West End is almost entirely devoid of new work and stuffed with musicals or comedies, with Alan Ayckbourn's Absent Friends, Noel Coward's Hay Fever and Singin' in the Rain all having opened within a few days of each other.

The job of subsidised theatre has traditionally been to bring on new writers and put on plays that challenge or inspire debate but would not be popular enough for a commercial theatre to stage.

Stressing the importance of venues such as the Hampstead theatre continuing to stage experimental plays like his – even if there were empty seats – Stephens told the Theatre Voice website: "It's urgent that state-subsidised theatres continue to stage work that is not going to find an audience … that's what state subsidy is for."

Hare said: "There is a homogenisation going on whereby theatres are becoming interchangeable. They are working to a public/private model plainly approved by the Arts Council, which is making them less risk-taking. The work of so many theatres now is centred around going into city boardrooms to get people to sign cheques. The Bush has gone from being a room over a pub to being a larger theatre which will have to have a public relations department, a development department – it's gone club class."

Ravenhill, writer in residence at the RSC, whose play Shopping and Fucking was an unlikely West End transfer in 1996, said the subsidised sector – where the West End hits War Horse, The Ladykillers and One Man, Two Guvnors all originated – now prized commerce over art.

"When I started working in the theatre in 1987, artistic directors used to programme plays knowing they wouldn't find an audience because they believed in them heart and soul," he said.

"I haven't heard anyone say that for the past 10 or 15 years. Now the discussion is: 'Is this going to transfer to the West End?' That's prized much more highly than, for instance, Sarah Kane's Blasted – which has never played in a commercial theatre – being taught, discussed and performed in hundreds of theatres all over the world."

Ravenhill said the role of subsidy had changed. "If a theatre is being subsidised, now you need to get in as many people as you possibly can and make sure they're as diverse as you can. You're being elitist if you put on something that plays to a house that's 20% full. Everything we put on is supposed to be for everybody, but most great plays aren't for everybody."

Nica Burns, vice-president of the Society of London Theatre, took issue with Ravenhill's remarks. "The years have shown the really fine stuff survives. Does he really believe there's a list of Beckett and Ionescos that are lying unformed and undiscovered because British theatre is not adventurous enough?" she said.

Burns acknowledged that in the subsidised theatre, "everyone's trying to fill up the arts cuts deficit by chasing the same philanthropic dollar, and they have to sell more tickets".

She said some venues were now programming similar work to West End theatres. "When I go to get rights to classic plays for commercial theatres, I'm often competing with subsidised sector theatres. There was a play that I regard as a great American classic, which I got the rights for and found that both the National and the Donmar were competing for it."

However, she said the overall theatre scene in London was healthy. "The Bush and the Royal Court are championing new writing and fringe venues are flourishing. Look at Theatre503, which started The Mountaintop, which won an Olivier award for best play, beating Jerusalem and Enron, and then played on Broadway."

Hare said some of London's subsidised theatres still programmed avant-garde work, citing the National's production of London Road, last year's verbatim musical about the Ipswich murders.

"I don't think it's as simple as saying experimental work is necessarily being driven into the ghetto. It needn't be if everyone holds their nerve, but frankly that's up to theatre managements, who I think are projecting on to the audience their own insecurities.

"I don't think there's any evidence of that in the audience themselves – the fear is coming from artistic directors and nervous boards."

The National was able to make up a £3m funding shortfall thanks to the profits generated by War Horse, which transferred to Broadway and the West End before being made into a Steven Spielberg film. However, Hare said it would be a mistake for other theatres to attempt to programme commercial fare in an attempt to emulate it.

"We're back to what Ken Loach said when David Cameron made his moronic remarks about the cinema [arguing for lottery money to be used to support mainstream films that would be commercial successes]," said Hare.

"I remember the artistic director of the National theatre tearing his hair out about War Horse and going round saying 'we're about to lose a million pounds'. If one could see the War Horses coming and make those calculations, it would be a much easier world to live in."