Scenes of rape and other forms of sexual violence will no longer be allowed in films classified for under-15s in a shake-up of the British ratings system.

The British Board of Film Classification (BBFC) will on Thursday publish new classification guidelines explaining in detail why films get the ratings they are given, from U up to R18.

By far the biggest change is around films that include sexual violence and pornography. The board said the changes were a result of a shift in public opinion over the last five years.

Under the new guidelines no film that contains depictions of rape or strong sexual violence will be allowed a rating below a 15.

An example of a film that would probably fall foul of the new system is the 2008 Keira Knightley period drama The Duchess. It was given a 12A rating but would now most likely get a 15 because of a scene where a woman is held down by a man on a bed and it is implied he is about to rape her.

“The feedback we have had from the public during the current consultation is that they don’t think there is any place for depictions of sexual violence at 12A at all,” said Craig Lapper, the head of compliance at the BBFC.

The guidelines also say depictions of sexual violence in 15-rated films must not be “detailed or prolonged”. That would mean the 2017 Taylor Sheridan thriller Wind River, which starred Elizabeth Olsen as an FBI agent tracking a killer on a Native American reservation, would in all likelihood get an 18 rating rather than 15 because of a violent scene towards the end of the film.

“The key message from the guideline consultation was heightened concern about sexual violence across the board,” said Lapper. “Although we operate very strict standards around sexual violence there was an appetite on the part of the public for us to be even more cautious than we are at the moment.”

He said people were also calling for a tightening up of rules around how much strong and crude sex references are allowed in 15-rated films.

An example would be Seth Rogen’s animated film Sausage Party, which culminates in a food orgy. Under the new guidelines it would have been cut or given an 18 rating, Lapper said.

The changes are the result of a public consultation exercise in which more than 10,000 people were interviewed. The BBFC conducts a consultation every five years.

As well as stricter guidelines on sexual violence, the BBFC said the public wanted greater age rating consistency over online channels, something which is not in the hands of the censors, although companies such as Netflix and Amazon do voluntarily submit content to them.

“There is no formal legal requirement and the ratings coverage on some platforms is a bit mixed,” Lapper said.

More than 90% of people in the consultation, and 95% of teenagers, said they wanted age ratings consistently applied to online content, the BBFC said.

Little or no change is being made to guidelines around swearing or non-sexual violence.

David Austin, the BBFC’s chief executive officer, said: “We know that people are more comfortable with issues such as action violence, if it’s in a way that they are expecting – such as a Bond or Bourne film.”