Based on Suetonius's Lives of the 12 Caesars, Caligula had been creating legal difficulties and controversy long before it was submitted to the BBFC for classification. Although made in 1976 the film was not premiered until 1979, by which time both the original writer (Gore Vidal) and the Director (Tinto Brass) had demanded that their names be removed from the film because they were unhappy with the direction that production and post production had taken. Amongst other things, it was alleged that additional scenes of real sex had been shot clandestinely and edited into the film against the director's wishes.

When a print of the film was finally imported to the UK in April 1980 for classification by the BBFC, it was seized and impounded by Customs and Excise as indecent and potentially obscene. After negotiations between the distributor, the BBFC and Customs and Excise, it was agreed that the print could be brought to the Board's premises in Soho so that the film could be edited to remove all footage that rendered the film at variance with Customs and Excise's standards and at potential risk under the Obscene Publications Act. Accordingly, the film was viewed at the BBFC by Customs and Excise, who indicated the footage to which they objected under their own legislation and guidelines, and by two lawyers, including a Treasury Counsel, who indicated all the scenes that might raise further legal difficulties under the Obscene Publications Act and other relevant UK laws.

The cut material included not only explicit sight of real sex but also scenes of violence and sexual violence that were felt to render the film potentially illegal. These included a scene of castration, a disemboweling, the rape of a virgin and sight of Caligula inserting his fist into a man's anus. Over eight minutes of footage were cut before Customs and Excise, the lawyers and the BBFC were satisfied that the film no longer contravened UK laws.

The process of editing to produce a legal version took a number of months, after which the distributors were asked whether they wanted to release this version in 'members only' cinema clubs, without a BBFC certificate, or whether they wanted to cut the film further in order to obtain an X certificate for distribution in public cinemas. Although the film was now considered legally acceptable, the Board's own standards for the X category were more stringent than the letter of the law, mainly because the Board also had to take account of the possible objections of local authorities on whose behalf cinema films were classified. The distributor indicated that they needed to release the film nationally in public cinemas and therefore the Board set about the task of making further cuts, mainly to genital detail but also to some aspects of violence which, although not illegal, exceeded the standards currently acceptable at the X category.

In total another three minutes of footage was cut in order to achieve an X classification, bringing the total amount of cuts made to satisfy Customs and Excise, the lawyers and the BBFC up to 11 minutes. In all, more than 50 separate cuts were made throughout the film. At this point the distributors asked if some visual material of a less explicit nature could be added to the film in order to allow some dialogue to be reinstated. The Board found the additional footage acceptable and finally, after six months of work, the film was granted an X classification. It was agreed that the footage cut for legal reasons would not be allowed to leave the BBFC and that, should the company wish to import more prints, they would have to cut these prints according to the same requirements before they were brought into the country.

Even after the significant cuts made to the film, press reports about Caligula being 'the most controversial film of the 80s' caused a number of local authorities to inspect the film for themselves. Although many local councils agreed with the Board that the cut version was acceptable for adult cinema audiences, a number of areas banned the film from cinemas in their area, including Cardiff, Blackpool and Portsmouth. Nonetheless, when the film finally opened it received poor reviews and relatively little protest.

In 1990, the uncut version of the film was submitted for video release in the hope of obtaining an R18 classification for sale in licensed sex shops. However, when it was pointed out to the distributor that explicit sight of real sex was not permitted at that time even in R18 videos, because of current interpretation of the Obscene Publications Act, they decided to submit a heavily reduced version of the film instead, edited in order to obtain an R rating from the MPAA (now the MPA) in the United States.

This version of the film was already missing nearly all of the material to which Customs and Excise, the lawyers and the BBFC had objected in 1980 (in fact a couple of brief shots of genital detail and sight of nurses cradling babies and phallic shaped bottles were reinstated and allowed to remain) but it was also missing a large amount of other footage. Some of this missing footage, previously allowed by the BBFC on film, had been considered unacceptable for an R rating in America, but a large amount of uncontentious narrative material had also been removed in an attempt to speed the film up. This had the effect of not only rendering the film comparatively innocuous but also destroying its continuity. In the event there was nothing in this very heavily reduced version to which the BBFC could object and the video version was accordingly classified 18 without any further cuts. Whereas the cut cinema version had run at over 2 hours 30 minutes, the video version ran at a mere 1 hour 40 minutes. In 1999 this heavily reduced version was released on DVD.

In 2008, the full uncut version of Caligula was resubmitted to the BBFC for DVD release. The passage of nearly 30 years had significantly diminished the film's impact and after careful consideration it was decided that it could now be classified 18 uncut. This decision accords with the BBFC Guidelines, which stated (at the time of classification) that 'At 18, the BBFC's Guideline concerns will not normally override the wish that adults should be free to choose their own entertainment, within the law'.

The uncut DVD version was rated 18 uncut for strong violence, sexual violence and strong real sex.

In 2024, a new edit of the film – Caligula: The Ultimate Cut – was submitted to the BBFC for a theatrical rating. Although longer than any previously classified version, many of the more visually explicit sex scenes present in the 2008 DVD version were removed or reduced for this submission. The BBFC classified this film 18 once more, this time with the content advice ‘strong real sex, sex references, nudity, sexual violence’.

Although there are scenes in Caligula that some people will find shocking, offensive or disgusting, the film does not contain any material this is illegal in terms of current UK law and nor does it contain any material that is likely to give rise to harm for adult audiences, most of whom will be well aware of its controversial reputation.