Health campaigners claim new rules to be published restricting the advertising of junk food to children on television will be too weak to halt the soaring levels of obesity.

Ofcom, the broadcasting regulator, is finalising a series of recommendations, which it will present to the government, on how to limit the number of adverts seen by children for products that contain unhealthy high levels of fat, salt and sugar.

But campaign groups such as the National Heart Forum, National Children's Bureau and food lobby group Sustain hit out yesterday at Ofcom's expected decision to reject widespread calls for all junk food commercials to be banned before the 9pm children's viewing 'watershed'.

'We are expecting a set of proposals that don't go far enough to tackle the crisis that we have in children's diet and reduce diet-related ill-health, such as obesity,' said Richard Watts of Sustain. 'We expect their advice to ministers to be disappointing and too weak to help play a part in solving these problems.'

Campaigners said implementing a ban on ads before 9pm, a move Ofcom has previously described as 'disproportionate', would be the single most meaningful route to help cut consumption of sweets, fast-food and fizzy drinks by the under-10s. Ofcom found that a ban would cost ITV, Channel 4 and Five about £100m a year in lost advertising and did not command widespread public support.

But Jo Butcher, the children's bureau's principal officer on development, said: 'Ofcom has failed to properly consider the 9pm watershed as an option for restricting junk food advertisements - in spite of the fact that health, consumer and children's groups remain convinced that the watershed is the only effective way to protect children from aggressive marketing of foods and drinks high in fat, sugar and salt.'

The TV watchdog is also thought to have rejected demands for a ban on brand advertising by companies such as McDonald's and for restrictions on such firms sponsoring programmes, as Cadbury does with ITV's Coronation Street.

Ofcom officials are putting the finishing touches to the new restrictions which they will bring in to achieve their stated aim of halving the number of ads for unhealthy products that children see. 'They'll be a lot tougher than many of the health campaigners think,' said a senior source at the Department for Culture, Media and Sport, which asked Ofcom to recommend ways to tighten the rules.

Ofcom is thought to have been looking in detail at either banning adverts before an earlier cut-off point, of 7pm or 8pm, or ending the broadcast of commercials during the traditional hours of children's programming, in the mid- to late-afternoons slots on weekdays and also on Saturday mornings.

The regulator has already announced plans to stop celebrities being used to advertise junk food during shows that are likely to attract younger viewers, a move that would prevent ads showing Beyonce promoting Pepsi and Gary Lineker doing the same for Walkers Crisps for example.

The Food and Drink Federation, which represents food manufacturers, has proposed a series of voluntary restrictions in an attempt to head off more stringent action, including an end to cartoon characters, celebrities and movie characters, or collectable gifts such as small toys, being used in adverts aimed at children. Food and drink commercials would be cut back to 30 seconds an hour all day on specialist children's channels such as GMTV Kids, the FDF added.

Government sources say Ofcom's proposals are one element of a wider strategy to limit the promotion of junk food and that a new voluntary code of conduct covering packaging, billboards and in-store advertisements will soon be forthcoming to complement efforts across Whitehall to end the increase in childhood obesity.