Health campaigners have reacted angrily to today's Ofcom claims that there has been a reduction in junk food TV ads aimed at children since new restrictions were introduced, renewing their calls for a pre-9pm ban to protect kids watching shows such as Coronation Street and The X Factor.

Richard Watt, the co-ordinator of the Children's Food Campaign at Sustain, said the organisation welcomed the report findings, but added that the "tiny falls" in broadcasters' advertising revenue meant the government had "no excuse" not to extend the restrictions and aid the fight against obesity.

Watt said: "Although we welcome this modest fall in children's exposure to junk food advertising, Ofcom's figures highlight just how far their rules fall short of what is needed. If we are serious about putting children's health first, we must protect them from junk food advertising before the 9pm watershed, because children watch most TV in the early evening."

He added that the relatively benign impact of the Ofcom restrictions, introduced in July last year, on broadcasters' ad revenues showed that the "cost of protecting children from junk food advertising is minimal", meaning an extended pre-9pm ban should be considered.

"Broadcasters have only suffered tiny falls in their revenue as a result of these rules and, for digital channels, this was offset by revenue rising from other kinds of advert. There is now no excuse for the government not to give our children the protection from junk food adverts they deserve," said Watt.

Betty McBride, the policy and communications director at the British Heart Foundation, added that the regulations do not make a big difference.

She said: "This report shows the majority of junk food ads are still getting through to kids. New rules on advertising during children's programmes won't make a big difference, as Ofcom admit the majority of ads kids see are between 6 and 9pm. The time has come for a full ban on junk foods ads before 9pm."

However, advertisers welcomed Ofcom's report, arguing that the industry had held up its end of the bargain to battle childhood obesity by changing its marketing practices.

"This news is very welcome to advertisers who have worked hard over the last three years to adapt their marketing strategies. And it comes hot on the heels of last week's positive compliance report on [junk food] ads from the [Advertising Standards Authority], further confirming the successful advertiser-regulator relationship," said Ian Twinn, the public affairs director at UK advertisers' body ISBA.

Culture secretary Andy Burnham said today that junk food TV advertising practices would remain "under review". Burnham, responding to Ofcom's review, broadly welcomed the findings while also warning that the government would continue to weigh all options in the battle against child obesity.

"I recognise the strength of feeling on this issue. That is why the regulations were strengthened, to ensure that food advertising to children is appropriate. These figures indicate that the regulations are having a positive impact. We will, of course, continue to keep the issue under review," he said.

Ofcom has always maintained that a total ban on junk food ads would have a "disproportionate" impact on ad revenues, costing broadcasters in excess of £140m per year.

By contrast, the media regulator has estimated that the cost of restrictions it has introduced on broadcasters could be as much as £39m per year.

According to Ofcom the hardest hit will be children's channels such as Cartoon Network and Nickelodeon with one unnamed kids' network estimating it could lose more than 15% of its total revenue.

The estimated impact on music channels is a loss of 1.9% of revenue, although one channel estimated it could lose almost 9% of its annual income.

Ofcom said that broadcasters could introduce a range of mitigating strategies, such as running more repeats or attracting new advertisers, which could see a reduction of overall industry revenue to between £17m and £27m with a "central estimate" of £23m.

The media regulator has estimated that the cost to the main commercial broadcasters – ITV, Channel 4 and Channel Five – will be between £7m and £12m a year.

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