When Watership Down was first released in 1978, the film’s director, Martin Rosen, insisted the image featured on its promotional poster should warn viewers that this was no cutesy cartoon about some bunnies.

“I reckoned a mother with a sensitive child would see a rabbit in a snare with blood coming out its mouth and reckon, ‘Well, maybe this isn’t for Charlie – it’s a little too tough,’” he has said.

Despite his concern – and the fact the animated movie includes blood-soaked visions, slavering, murderous predators and a rabbit having its throat ripped open – British film classifiers awarded it a U rating, suggesting it was suitable even for very young children.

Early viewers did not agree – and there is an entire generation for whom the gentle opening bars of Art Garfunkel’s theme song Bright Eyes will always evoke horrifying visions of bloodthirsty lapine massacre.

Now, 45 years later, the British Board of Film Classification has reviewed its opinion of a movie that has been called “a one-way ticket to post-traumatic stress disorder” after a resubmission. It has been reclassified as a PG, on account, the organisation said, of its “mild violence, threat, brief bloody images and bad language”.

“Whenever a distributor resubmits a film with an existing BBFC rating to us, we review it under our current guidelines,” the board said in its annual report. “This sometimes means we may reclassify the film at either a higher rating or a lower rating than it was under previous guidelines.”

Parts of Watership Down, the report acknowledged, could be “distressing”. It said: “In their exile, the rabbits meet various challenges, some of which result in bloody bite and claw injuries caused by animals fighting … In one scene, a bird tells one of the rabbits to ‘piss off’.

“When we viewed the film under the current guidelines we reclassified it PG in line with our current policies for violence, threat, injury detail and language.”

PG, which stands for parental guidance, is awarded to films that are for general viewing, “but some scenes may be unsuitable for young children”, according to the BBF. “A PG film should not unsettle a child aged around eight or older.”

The movie, which was adapted from the bestselling novel by Richard Adams and used the voices of John Hurt and Richard Briers, was passed for very young children in 1978 because of its cartoon format, the board said at the time of its release. “Animation removes the realistic gory horror in the occasional scenes of violence and bloodshed, and we felt that, while the film may move children emotionally during the film’s duration, it could not seriously trouble them once the spell of the story is broken,” it added.

But after an Easter TV screening on Channel 5 prompted renewed terror in 2016 (“Hey kids, let’s watch dead Easter bunnies!” tweeted one annoyed viewer), the BBFC’s director, David Austin, said the film would not be given a U rating nowadays, adding: “Standards were different then.”

He said the film continued to provoke complaints every year about its rating, decades after its release. The BBFC updates its guidelines every four to five years to “continue to meet the expectations and values of people across the UK”.

Another film to have its U rating raised to a PG this year is Star Trek: The Motion Picture, the first movie spin-off of the sci-fi TV series. The 1979 film, which was directed by Robert Wise and starred William Shatner and Leonard Nimoy, was reclassified because of “brief mild horror and sex references”.

In total, the BBFC classified 1,057 cinema films, 5,527 video submissions and 3,649 online submissions in 2022, the report said. The most common rating was 15 for both online and physical media submissions, with 42% of all classified content falling into the area.

The BBFC has licensed 33 brands and services to carry its age ratings and data including the streaming firms Netflix, Amazon Prime Video and Apple TV+.