It has been "aggressively" and "unfairly" marketed at young audiences, sending the message that violence is acceptable, according to the British Board of Film Classification (BBFC).
The BBFC had come under fire for giving the blockbuster a 12 certificate, meaning no-one under that age could see it.
At least three local councils have used their powers to change the rating and let younger viewers into cinemas.
But the BBFC has defended its stance, saying the level of violence almost merited the higher certificate, a 15.
'Clear message'
"Spider-Man is possibly the most violent film which is aimed at a young audience that the BBFC has classified," a statement said.
Do you think Spider-Man is too violent for children?
"The violence is set in a modern urban setting with a clear message that the use of violence is the normal and appropriate response when challenged.
"The Board does not believe that this is the sort of message to be sending to young children."
It carried out extensive research, which it said showed that parents were "very concerned" about the levels of violence.
"Hollywood has carried out an aggressive world-wide marketing campaign aimed at young children when the film is not suitable for them."
The film's makers, Sony Pictures, were unavailable for comment.
Spider-Man is played by Tobey Maguire, who is involved in an ongoing battle with his enemy, the Green Goblin, played by Willem Dafoe.
Revise
It broke US box office records when it was released there in May, and reaches UK cinemas on Friday, 14 June.
A number of cinemas have lobbied local councils, who have the power to revise the BBFC's rating for cinemas in their area.
North Norfolk and Breckland District Councils, in East Anglia, have changed it to a PG, which means unaccompanied children of any age may watch.
Tameside council, near Manchester, has denoted it a "PG-12", meaning children under 12 could only be admitted if they were with an adult.
They have taken their lead from US regulators, who gave it a PG-13 classification.
John Wilkinson, chairman of the Cinema Exhibitors' Association, told BBC News Online that the reason cinema operators were lobbying councils was so they were not harangued by angry parents of children who wanted to see it.
"Cinemas aren't doing it for the money," he said. "The real stimulant is - this has been publicised highly and we know that parents have made the decision to take their children to see this film."
Many cinemas faced a lot of "hassle" from parents whose young children wanted to see 1997's Titanic, which was a 12 - and did not want to repeat the experience, Mr Wilkinson said.
The BBFC said it was working towards a new PG-12-style rating - which would shift the responsibility to the parents - but no final decision had been made or date set.
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