The censor is to take a much more relaxed attitude to sex in 18 certificate films in the first formal loosening of Britain's relatively prudish screen code.

The British Board of Film Classification said yesterday it would now "only rarely" cut explicit scenes after a six-month consultation process found the public would rather make up their own minds about what was acceptable.

But the new liberal mood to sex - which extends to some degree to violence - disguises a far more restrictive line on drugs and bad language in films aimed at younger audiences.

In the commercially crucial 15 category - where the board has frequently been at loggerheads with film-makers who view the 18 certificate as a box office death sentence - strong language will "only be rarely acceptable".

After the recent furore over the "ludicrous" 18 rating given to The End of the Affair, there is to be a more permissive attitude to sex in 15 certificate films.

Significantly, however, the relaxation applies only to its depiction in "responsible, loving and developing relationships". Casual sex, the board warns, should be "handled responsibly".

Last night Pact, the producers' alliance which represents the bulk of British film-makers, gave the changes a cautious welcome but claimed there was a "touch of moral panic about them".

The guidelines show their teeth in the depiction of drug use. All mention of them in U films and videos will be banned, references severely curtailed at PG, 12 and 15 levels, while scenes akin to those in Trainspotting which give "instructive detail" in 18 certificate films will be cut.

Robin Duval, the BBFC's director, revealed that if Quentin Tarantino's Pulp Fiction - in which John Travolta played a hitman with a heroin habit - came before him now, he would consider cutting it.

Mr Duval admitted that with film-makers like Tarantino and his many imitators there were "difficult calls to be made between promoting and portraying violence". He said the board was now prepared to "take extreme action" against films which promoted such cruelty.

The board's president, Andreas Whittam Smith, who is seen as steering a more conservative course than the previous regime under James Ferman, said he had worries about the blurring of the line between "comic or stylised violence" and scenes which revelled and gloried in it.

He said he aimed to alter the guidelines every few years in line with changing attitudes, but warned that this "does not mean they will shift in the same direction".

The new rules were "not a sea change" but simply a clarification of the way decisions were now made. Even so, Mr Whittam Smith admitted the BBFC would still find itself "in a pickle" with the growing trend towards "naturalistic" sex scenes in arthouse films like Romance and Seul Contre Tous. He said the board would never be able to pass anything which conflicted with the Obscene Publications Act.

But he made a stout defence of the board's decision to be less proscriptive about sex scenes in 15 films.

"We have the toughest guidelines in the world on film already and still we have the highest level of teenage pregnancy," he said. "We have the power to intervene and cut which they do not have abroad, we have mandatory certification, and we also control the video market."

The BBFC's decision to err on the side of tolerance at 18 comes after it was forced to relax the rules on sex videos after being defeated in the high court earlier this year. The explicit R18 films are available only through licensed sex shops.

But the new guidelines themselves follow a study, Sense and Sensibilities, commissioned by the board a year ago into public attitudes.

The research showed 56% of those surveyed thought young people used bad language because of what they heard in films. Nearly half, however, thought adults should be allowed to watch real sex on the screen.

Censorship ... and sensibility

Sex
The End Of The Affair - and Ralph Fiennes' offending buttocks - will now pass as 15 rather than getting an 18 rating, while the difficulty in passing "real sex" scenes in arthouse films like The Idiots and Romance are now eased because they can be "exceptionally justified by context".

Violence
The more liberal line on the 18 rating means The Fight Club, in which Brad Pitt and Edward Norton starred as corporate suits who relaxed through bare-knuckle boxing, would now pass uncut. Last House on the Left, the only film the BBFC refused to certify this year, would still not pass because of its "incendiary mix of sex and violence".

Drugs
Several scenes in Trainspotting which could be interpreted as giving "instructional detail" on drug-taking could now go, while "if Pulp Fiction were to come to us now, we would look at that scene where we see John Travolta in a car high on heroin with a little more concern".