Black Swan draws the most complaints of 2011 as Natalie Portman and Mila Kunis' lesbian sex scene leaves cinema goers hot under the collar

British cinema goers were left hot under the collar over a lesbian sex scene in ballet thriller Black Swan last year, swapping their tutus for tuts as the film drew the most complaints of 2011.

Movies including The Hangover – Part II, Twilight - Breaking Dawn - Part 1 and Sucker Punch were also the focus of public anger last year, according to the British Board of Film Classification, which announced its annual report today.

The certification body added it was to commission new research into sexual violence in movies and revealed its desire to take a tougher stance on content that is exempt from classification such as music videos and documentaries.

The 15-rated Black Swan, the tale of a ballet dancer’s mental collapse, generated 40 complaints. The BBFC said that while that number of complaints was high by its own standards, it was “proportionately very small indeed” for the 2.7 million people who saw it in cinemas. The film was met with huge critical acclaim and secured star Natalie Portman the Oscar for best actress.

The “standout issue” for most of the cinemagoers who complained, was a lesbian sex scene between Portman and co-star Mila Kunis, in which “one female character performs cunnilingus on another”.

While the scene was not particularly explicit – the BBFC described it as “visually discreet” – some correspondents "felt it was pornographic in nature”.

David Cooke, director of the BBFC, said: “It is a fairly strong scene, but it’s not anything like above the line. We felt Black Swan was in the boundary of a sex scene for a 15 rated film.”

That the sex scene was between two women caused problems for some “who argued that portrayals of homosexual activity should either be restricted to the 18 category or not shown at all,” the report said. Mr Cooke shot back: "We have a clear policy in the guidelines that we don’t differentiate or discriminate whether it’s a straight or gay or lesbian scene."

He added: “I think a big thing was confounded expectations” as people expecting a film about ballet may have been shocked by the intensity of the thriller storyline.

Viewers complained about the the crudity of the second Hangover movie, while some felt the emergency Caesarean section performed on the lead character in Twilight was "gory and distressing".

The most complained about movie in the past decade, which was fuelled by a media campaign, was the second in director Christopher Nolan’s Batman trilogy: The Dark Knight. The BBFC does not expect similar levels of complaint for the third in the series The Dark Knight Rises, which opens next week.

The classification board was involved in several high profile battles with film makers this year. Human Centipede 2 (Full Sequence) was initially refused theatrical release but after the case threatened to end up in court, cuts were agreed between the body and the filmmakers.

The Bunny Game, however, was refused classification altogether. The board felt the story of a trucker torturing a prostitute crossed the line of glamorising sexual violence.

The BBFC takes the issue of sexual violence very seriously and is conducting new research into films’ handling of “sadistic, sexual and sexualised violence, mainly against women” to help inform its future decisions on certification.

Separately, the board revealed that it still receives letters of complaints about classics such as Watership Down – that it is too upsetting – and The Railway Children, as one correspondent suggested it might encourage young people to walk on railway tracks.