The 1978 animated film Watership Down has been re-classified to a PG due to its "mild violence, threat, brief bloody images and bad language".
The movie is among the classic titles to have had their age ratings raised, along with the original Star Trek, according to the British Board of Film Classification's (BBFC) annual report.
After being resubmitted, the ratings were raised by the organisation, it said, in order to ensure they "remain in step with societal standards".
Watership Down, which is based on the novel by Richard Adams, tells the tale of a group of rabbits who leave their burrow in search of a new home.
Their quest brings them into contact with a battle-scarred rabbit called "General" Woundwort, as well as a gull called Kehaar who tells another character to go away using an expletive.
The film - which features the voices of Sir John Hurt and the late Richard Briers - received a U [Universal] rating on its initial release for its "very mild language, mild violence and threat".
Star Trek: The Motion Picture - the sci-fi hit first released in 1979, starring William Shatner and Leonard Nimoy - was also originally deemed "suitable for all".
But it too now requires a PG [parental guidance] sticker for containing "brief mild horror and sex references".
'Expectations evolving'
The BBFC report noted: "Whenever a distributor resubmits a film with an existing BBFC rating to us, we review it under our current guidelines.
"This sometimes means we may reclassify the film at either a higher rating or a lower rating than it was under previous guidelines."
It referenced a "distressing sequence" in Watership Down, explaining: "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 '[pee] 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," it added.
BBFC director David Austin told BBC Radio 5 live in 2016 the film's violence and language was "arguably too strong" for it to be rated U today.
His comments followed complaints over the film's content after it was aired on Channel 5 on Easter Sunday earlier that year.
The movie returned to the small screen with a BBC adaption in 2018, featuring a voice cast including James McAvoy, John Boyega and, Olivia Colman.
The BBFC updates guidelines every four to five years in order to "continue to meet the expectations and values of people across the UK".
The next consultation is scheduled for this year, with any required changes coming into effect by 2024.
Natasha Kaplinksy, president of the board, said it was currently consulting with over 10,000 people across the UK "to explore how audience expectations are evolving".
"We will then reflect these changes in our classification guidelines, which set the foundation for all of our age rating decisions, including when older films are resubmitted and receive a new classification."