Until now, the findings have been secret, with ministers and public health bodies refusing to publish the report until the Government publishes its strategy on childhood obesity, due in January.

The head of the Commons health select committee suggested the report had been suppressed because its findings were “inconvenient” and likely to support a sugar tax, which ministers have opposed.

Under questioning from MPs, public health officials yesterday disclosed that the review found that “universally all the evidence shows that tax does decrease purchases”.

Dr Alison Tedstone, PHE director of diet and obesity said: “PHE does see there is a role for a fiscal approach in reducing sugary drink consumption. The higher the tax increase the greater the effect,” she said.

“The point of the tax is to nudge people away from purchasing these things towards purchasing things that are more consistent with a healthy balanced diet,” she said.

But she said it was not the top priority for the agency, which thought that restrictions on promotion and advertising of foods to children, and reformulating foods to reduce their sugar content, could have a larger effect.

“We think there could be bigger impacts from getting a handle on promotions, and of getting a handle on the deep, consistent advertising our children are exposed to on unhealthy foods,” she said.

“PHE are advising that promotions need to be restricted and rebalanced if we want to reduce sugar consumption,” she said.

In particular, she singled out the rules about junk food advertising in children’s TV as not going far enough, in only preventing branded cartoon characters from being deployed.

As a result, companies which could not use figures such as “Dumbo” to market products could invent their own characters – such as the Coco Pops monkey, used by Kelloggs, aimed at attracting children.

“So Dumbo can’t be [used to promote unhealthy food] but things like the Coco Pops monkey can be. Yet the evidence is that things like those Coco Pop monkeys do engage children and affect food preference and choice,” she told MPs.

The health official said tackling obesity among children would not be sufficient to tackle Britain’s bulging waistlines, and said adults needed help too.

Dr Tedstone said restrictions should also be introduced on portion sizes for some junk foods and raised concern about the trend for “supersizing”.

“I’m very worried that we’re beginning to see practices that we commonly see in the States coming into the UK, for example we’re seeing bottomless cups in some restaurants,” she said, while also criticising the growth in sizes of other products, such as crisps.

“Bags of crisps are substantially larger than they were 15 years ago,” the official said. “I don’t think many of us need those crisps at the bottom of the packet.”

The review also concluded that the food industry should be put under pressure to reduce the amount of sugar in its products, in the same way that foods have been reformulated to reduce SALT content, Dr Tedstone said.