Junk food adverts have been banned from television when programmes aimed at young children are being shown.
The ban covers TV programmes aimed at young children
The new rule means adverts for food and drinks high in fat, salt or sugar cannot be broadcast around shows aimed at four to nine-year-olds.
Some TV channels and advertisers fear the ban - introduced over fears about childhood obesity - is too harsh.
But Dr David Haslam, from the National Obesity Forum, said he believes the ban does not go far enough.
He told the BBC's Radio Five Live: "If you go into a supermarket there are, once again, sweets and chocolates at eye-level for a child at the till.
"Now we had a phase when that was banned and we didn't see it, but now they are creeping back again in some of the major stores - well, that's something that should be stopped instantly.
"It's all very well banning advertising of the food to kids, but kids are still going to be eating the food because there's point-of-sales advertising and there's the pressure that's put on the parents to go out and get a fast food burger."
From 1 January 2008 the restrictions will be extended to TV shows aimed at children up to 15 years, as well as adult programmes watched by a large number of children.
Dedicated children's TV channels are allowed to phase in the restrictions affecting four to nine-year-olds from 1 April, but must have the full ban in place by 1 January 2009.
An spokeswoman for Ofcom - which introduced the ban - said: "The new rules are one of a wide range of measures aimed at tackling child obesity.
"By reducing the exposure of children to advertising for products that are high in fat, salt and sugar, the advertising restrictions will contribute to wider efforts to promote healthier diets amongst children."
TV channels and the Broadcast Advertising Clearance Centre (BACC) are responsible for ensuring commercials comply with the new rules, while Food Standards Agency guidelines will be used to decide which products are subject to the rules.
Hamish Pringle, director general of the Institute of Practitioners in Advertising, told BBC News 24 he was confident that the ban would not affect the industry too much.
"Yes, advertisers have lost the opportunity to communicate directly to children, [but] they still have the opportunity to talk to their parents," he said.
"Advertisers can still compete with each other, they can still bring in new products, and I think that's a good thing for the economy as a whole."