BORIS Johnson has been accused of "turning back the clock" on the fight against childhood obesity with his pledge to review so-called sin taxes.

The frontrunner in the race to become the next Prime Minister came under heavy criticism after promising to review the taxes, specifically on sugar and salt; the review will not cover tobacco and alcohol.

The former Foreign Secretary vowed not to introduce new sin taxes or increase current levels unless the review concluded they were effective and did not unfairly hit those on low incomes.

The sugar tax on soft drinks has been celebrated by the Treasury for shrinking the amount of sugar in the national diet by 45 million kilograms a year and for raising millions in sports funding.

Mr Johnson’s commitment coincided with a warning from Cancer Research UK that obesity caused more cases of some cancers than smoking.

It also came as UK Health Secretary Matt Hancock, his key ally, prepared to publish a Government Green Paper recommending the extension of the sugar tax to milkshakes.

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Shirley Cramer, for the Royal Society for Public Health, said the organisation was "seriously disappointed" by the announcement and praised moves to tackle the childhood obesity epidemic.

"One of the successes of the last Conservative Government was the introduction of the sugar levy for soft drinks. The evidence shows that the sugar levy has worked.

“We should be building on the success of the sugar levy, not turning back the clock on the progress that has been made so far," she added.

Michelle Mitchell, Chief Executive of Cancer Research UK, praised the successes of taxation, including the sugar tax introduced in April last year.

"They have been highly effective in bringing down smoking rates to record lows, including within deprived communities," she declared.

A snap poll suggested the majority of adults backed higher taxes on food and drinks that were high in sugar, salt and fat to tackle health issues.

Some 55 per cent of the more than 2,800 polled approved of the measures.

The Tory membership, which Mr Johnson must win over to become Prime Minister also appeared to back the taxes with 54 per cent voicing their approval in the poll.

Jeremy Hunt called on his leadership rival to announce a plan to fix the obesity crisis.

"We have the second highest number of obese young people anywhere in Europe and we do need to have a solution to this," declared the Foreign Secretary.

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"So, the people who want to scrap these taxes need to say what is their plan because it's terrible for the young people involved."

Jonathan Ashworth, the Shadow Health Secretary, claimed Mr Johnson’s move to "water down" the obesity strategy was "extraordinary".

He said: "Boris Johnson has shown that his priority is representing the interests of his wealthy supporters, with no concern for the health and well-being of the general public."

Tory backbencher Steve Brine, the former Health Minister, was also critical of Mr Johnson’s proposal, branding it “transparent dog-whistle politics”.