The media regulator yesterday stood firm in the face of sustained lobbying from food manufacturers, introducing a ban on junk food advertising during all programmes aimed at children under 16. But after delivering the final word in a protracted and controversial process, Ofcom immediately came under fire from health groups accusing it of watering down "already weak" proposals by giving broadcasters longer to implement them. They vowed to continue their fight for a mandatory 9pm watershed on advertising for food high in fat, salt and sugar.

Under the new rules programmes with a higher than average proportion of viewers under 16, potentially including popular shows such as The Simpsons, Hollyoaks and Skins, will be affected. But Ofcom said broadcasters would be able to phase in the changes gradually, with restrictions aimed at under-10s introduced from April and those aimed at under-16s from the start of next year. Full implementation will not be required by dedicated children's channels until the end of next year.

"They have failed to protect children from junk food adverts or help solve the childhood obesity crisis," said Richard Watts, coordinator of the Children's Food Campaign for pressure group Sustain. "This is hardly surprising given their insistence on trying to balance the physical health of kids against the economic health of broadcasters."

Sue Davies, chief policy adviser for the consumer association Which?, claimed Ofcom had "let children down".

"They will still allow cartoon characters to promote unhealthy foods during Coronation Street and Dancing on Ice, so how will this make a difference?" she said. "With an escalating obesity crisis we can't afford to wait another couple of years before stepping in."

Ofcom has estimated that the new regulations, which will particularly hit digital channels with a disproportionate number of young viewers such as Sky One, MTV and E4, will cost broadcasters £39m a year. ITV has warned that they could put the future of domestically produced commercial children's programming at risk and the future health of the sector is the subject of a separate Ofcom review.

Celebrities and licensed characters from films and television shows will no longer be allowed to advertise junk food aimed at primary school-aged children or younger. But manufacturers can still use their own characters such as Snap, Crackle and Pop and Tony the Tiger.