He says it is elitist and moribund; its audiences are patronising and do not like young people volubly enjoying themselves; the critics are out of touch. Real kudos and glamour only impinge when a star "getting bored with film" decides to tread the boards - Ewan McGregor, Nicole Kidman, Juliette Binoche. But, warns Welsh, "do not hold your breath waiting for their next play".
An adaptation of Trainspotting was a success on the stage; and Welsh's latest play, You'll Have Had Your Hole, premiered in Leeds and had its London West End opening to a young and starry audience on Wednesday night.
But, writing in The Stage newspaper today, Welsh pours scorn on theatre and its audiences. He says "theatre is seen as boring, pompous and second- rate" by most practitioners and appreciators of other art forms such as cinema and music. Trainspotting brought new people into the theatre, he says, but this was not seen as a good thing.
"The patronising attitude displayed towards members of the audience for having the audacity to enjoy the play was turned into outright hostility when my play You'll Have Had Your Hole premiered in Leeds.
"Sadly this was not surprising. Theatre is posher and older than most mediums - this vibe generally hits you as soon as you walk into one. The soporific content of the majority of West End plays and the cricket Test ambience of the theatre seems essentially designed to keep a younger, hipper crew away. This seems to hold for performers as well as audiences."
You'll Have Had Your Hole - a story of a kidnap of a gangster that includes anal rape, torture and drug-taking - was described by one critic as "the most obnoxious and contemptible" play he had seen.
However, Welsh discloses that he was happy to use negative reviews by theatre critics to publicise the play as he was "working on the premise that condemnation from the out-of-touch is as valid an endorsement as praise from the hip".
One promoter did back off, he reveals, because of the reviews. That was not the fault of the critics, "but of the spineless, tunnel-visioned promoters and administrators who want to service an expanding market rather than expand it". The play's London premiere was at a rock concert venue, the Astoria.
Welsh goes on to say that theatre and its audiences today would not be endorsed by Shakespeare. He declares: "Shakespeare would have empathised with the lager brigade, staggering in to see Trainspotting. His audiences were not just old and bourgeois, they were critical, clued-up punters who would stand around drinking, and give the actors a bit of verbal if they were bored... They were part of a much more vibrant theatre scene than the moribund nonsense that passes for one now."
Accusing theatre of "cosy elitism", Welsh concludes: "It might be cool if theatre could appeal to some in their teens, twenties and thirties who did not go to Cambridge. To do this, however, we are going to have to encourage stage plays that deal with troubling, contemporary material, and then allow them to be appreciated, hated and enjoyed by the people who get it."