Yet as Robin Duval, director of the BBFC, notes in his ruling: "[T]he violent content of Fight Club is rather less than many other films which have passed entirely without public or media concern."

The casual observer may find it difficult to understand what is permitted and what is not.

The board does not entirely rely on written guidelines.

The only absolutes it must adhere to are those set down in law. These include the Obscene Publications Acts (no material which is likely to be depraving or corrupting), laws on blasphemy, criminal libel and those governing the treatment of animals and use of children.

Beyond that, it bases decisions on a system of precedent. The aim is to ensure consistency.

Romance: The most sexually explicit film given a cinema release

In fact the board has no legal status. Local authorities have the ultimate say on whether a film can be shown, although in most cases they are happy to defer to the BBFC. It also classifies videos, DVDs and video games.

The tenets of BBFC doctrine are context, treatment and the intention of the film-maker.

"Virtually any theme can be accepted if the treatment is responsible," say the BBFC guidelines.

"The same images may be acceptable in one context but not in another."

Fight Club, starring Brad Pitt and Ed Norton

This approach is applied when judging how to classify a film - U, PG, 15 etc - and whether a film should be cut or banned outright.

The process of vetting a film is therefore highly subjective and might explain why the BBFC, for the first time in its 87-year history, is consulting the public before settling its new guidelines.

It says the majority of the cuts fall into one of the following areas:

• Sexual violence - there is a strict policy on material that seeks to exploit of glamorise rape or sexual assault.

  In June this year, the 1971 Sam Peckinpah film Straw Dogs was refused a video certificate, citing the double rape of the character Amy. The rapes were considered violent and erotic. The board also took issue with the fact "Amy comes to enjoy being raped".

• Emphasis on violence or sadism - "most notably in scenes of torture where violence is presented as prolonged enjoyment".

  In the judgement on Fight Club, Mr Duval said the violence was "excessively sustained" and there was "an indulgence in the excitement of beating a defenceless man".

• Sexual explicitness which falls foul of the obscenity law. This was at issue in the recent cinema release of the French film Romance.

  The film, released uncut this autumn, contains an unprecedented amount of explicit sex, including scenes of fellatio, bondage and - for the first time - an erect penis.

  Defending its decision, the board called the film philosophical rather than pornographic and offered "insights about the female condition".

• The board also takes a harder line on videos over cinema releases, on the basis they can be seen more easily by children and scenes can be played over and over again.

  Last year it upped the video classification of the Paul Verhoeven film Starship Troopers from 15 to 18, which it had been in the cinema. However, it also lowered the teenage horror movie I Know What You Did Last Summer from 18 to 15, saying the effects were "considerably diminished on the small screen".

Yet despite the fuss over other controversial releases in recent years, including Reservoir Dogs, Natural Born Killers and Crash, the BBFC refuses certificates to only a handful of films.

In the 1980s only 16 movies were refused under its guidelines and as attitudes to sex and bad language - though not violence - relax, the trend continues that way.

The E-cyclopedia can be contacted at e-cyclopedia@bbc.co.uk