The Human Centipede, a 2010 horror film in which a scientist stitches kidnap victims together, was proudly touted as "the most horrific film ever made".

But its Dutch director, Tom Six, may have gone too far in the follow-up, because the British Board of Film Classification (BBFC) has denied The Human Centipede II (Full Sequence) an 18 certificate for fears it poses a "real risk" to cinemagoers.

The BBFC refusal means it cannot be legally supplied anywhere in the UK – even on DVD or download.

In the sequel, a man becomes erotically obsessed with a DVD copy of the original film – in which the victims are surgically stitched together mouth to anus – and decides to recreate the idea.

The film then focuses on his fantasies and the torture he inflicts. One scene involves him wrapping barbed wire around his penis and raping the woman at the end of the centipede, having become aroused by the sight of his victims being forced to defecate into each others' mouths.

The BBFC described the central plot of the film as the "sexual arousal of the central character at both the idea and the spectacle of the total degradation, humiliation, mutilation, torture and murder of his naked victims".

It took the rare move of refusing to classify the film and explaining that no amount of cuts would allow them to give it a certificate.

"There is little attempt to portray any of the victims in the film as anything other than objects to be brutalised, degraded and mutilated for the amusement and arousal of the central character, as well as for the pleasure of the audience," the BBFC said.

The board also said The Human Centipede II may breach the Obscene Publications Act, and "poses a real, as opposed to a fanciful, risk that harm is likely to be caused to potential viewers".

The BBFC says it has withheld certification for 28 films, including the Centipede sequel, since 2000. The previous example was Grotesque, a 2009 Japanese horror film whose premise was likewise deemed dangerously offensive.

"The chief pleasure on offer," said BBFC director David Cooke at the time, "seems to be wallowing in the spectacle of sadism (including sexual sadism) for its own sake."

Grotesque's director, Koji Shiraishi, responded warmly to the decision, saying he was "delighted and flattered ... since the film is an honest, conscientious work, made to upset the so-called moralists".

Last year the organisation demanded an extensive edit totalling 49 cuts to A Serbian Film, another hardcore torture movie, before it was passed with an 18 certificate.

But the publicity surrounding the BBFC's action was feared to have increased the film's reach.

Similar fears surrounded the release of the first Human Centipede film, whose content was vigorously defended by Six in interviews.

The director also promised then that part one would be "My Little Pony compared with part two".

• This article was amended on 10 June 2011. The original contained some references to a BBFC ban, including a line saying the BBFC had banned 11 films in its history. This has been corrected.