A new film rating, the 12A, will replace the 12 certificate, allowing children to see more graphic films at the cinema if accompanied by an adult.
BBC News Online looks at what it will mean for parents and children.
How will the 12A rating work?
Children under the age of 12 will be allowed to watch 12A films at cinemas if they are taken by an adult.
An adult is classed as someone over 18. Children under 12 could not watch films with the old 12 rating at cinemas at all before.
What types of films will children now be able to watch?
The 12A rating will be given to films that would have previously been 12s - films with some "mature themes" that are deemed suitable for young teenagers.
Graphic scenes of sex and violence will be largely avoided. But the films will be more graphic than the PG and U-rated ones that children under 12 have previously been able to watch.
According to the British Board of Film Classification (BBFC), nudity is allowed, but will be brief and discreet if shown in a sexual context.
Violence must not dwell on detail, with no emphasis on injuries or blood, while strong swear-words should be rare and justified by context.
Will children always have to be accompanied by an adult?
Yes. The adult must go and see the film with the child, not just buy their ticket or drop them off at the cinema.
How will adults know which films are suitable?
Film posters and adverts have started to include more factual information about the film's content, such as how much sex and violence to
Does the accompanying adult have to be a parent?
No. The BBFC is taking the view that all adults are responsible, but said cinemas can refuse entry if staff think they are not.
Who will check that the children are with an adult?
As with the other ratings - 15, 18 etc - the responsibility for enforcing the 12A will lie with the cinema.
Will there now be any age limit at all?
No. As long as they are with an adult, children of any age will be allowed into the cinema. But the BBFC said it does not recommend taking very young children to see 12A films.
Will this change apply to videos as well as cinemas?
No. The old 12 rating will continue to be used for videos.
Video ratings apply to the people who buy or rent videos, not to those who watch them, so the BBFC said an "advisory" rating would be irrelevant.
Why have they decided to make this change now?
Most of Europe, the US, Australia and Canada use similar systems - and the BBFC said 70% of the UK public are in favour of this change.
The BBFC said the impetus to make the change came from parents writing to complain their children were being denied entry to films they wanted to see.
The move was not in response to the controversy over Spider-Man in June, when some local councils overruled its decision to make the film a 12, the BBFC said.
Pressure from the film industry to let more paying customers through the turnstiles was also not a contributing factor, it added.