Scotland Yard is today investigating a controversial new play after it apparently showed live sex acts in a London theatre.

The review XXX, which had its premiere last night at the Riverside Studios in Hammersmith, featured some of the most realistic sex scenes ever seen in a British theatre.

At one point a man from the audience was dragged on stage where a female member of the cast appeared to perform oral sex on him.

A Home Office spokeswoman said that sex in public is a criminal offence, meaning the theatre and the members of the cast could be prosecuted.

It is up to the police to decide who is ultimately responsible for the play. The spokeswoman added: In terms of law it does not make any difference if it was a member of the cast or a member of the audience who was involved. Sex in public is an offence and they could be prosecuted.

The performance by the fourstrong Spanish troupe La Fura Dels Baus - two women and two men - is based on the Marquis de Sade's 18th-century book Philosophy In The Bedroom.

La Fura's director Alex Olle said today that the audience member involved in the show was a "plant" placed by the company and that his penis was prosthetic.

However, the Home Office said even the use of a prosthesis could lead to a prosecution.

Olle said: This is the only country where people have asked whether it was real. Since La Fura was founded 25 years ago we have wanted to shock people so they can open their minds. For me this reaction is funny - and it is good for our ticket sales. Olle produced the prosthetic penis used last night as evidence, although the man himself, when questioned immediately after the show, had insisted he was not connected with the show and had genuinely been engaged in oral sex on stage.

However, Evening Standard reporter Tim Cooper said: Last night I was convinced that I had seen a real sex act taking place on stage - and everyone else in the audience thought so, too. Today the company produced the prosthesis and said the act had been faked. While I am willing to believe them I have to say that the performance last night was the most convincing I have ever seen.

Olle said La Fura recruits volunteers for the audience participation from theatre schools in Spain and that others would take part in future performances. If a genuine audience member joins us on stage, as has happened in other European cities, the performers will not engage in sexual acts with them, he added.

The show featured scenes of anal, vaginal and oral sex, enacted on stage but apparently simulated with the help of various sex aids - although many of the audience believed they were watching genuine live sex.

The video screen behind the stage depicted genuine hardcore sex scenes throughout the show, from the opening clip of a woman defecating onto the camera lens. Others included explicit scenes involving bestiality, double penetration, ejaculation and graphic footage of genital torture and mutilation.

Outside the theatre a handful of demonstrators handed out leaflets protesting that the show was lowering the barriers of taste and decency, and demanding tougher obscenity laws.

Standard art critic Brian Sewell said the show should be allowed to continue no matter what happened on stage. He said: I don't see why these things should not be done on the stage. If you are offended all you have to do is get up and walk out. That is the only kind of censorship that is honest."