Boris Johnson has announced a wide-ranging review of “sin stealth taxes” just days before his high-profile supporter Matt Hancock, the health secretary, is due to publish a green paper advocating extending the sugar tax to milkshakes.

The policy green paper called Advancing our Health – Prevention in the 2020s, has been circulating among cabinet ministers this week and is due to be published in the next few days.

A draft seen by the Guardian includes widely trailed plans to extend the sugar tax to “sugary milk drinks … if the evidence shows that industry has not made enough progress on reducing sugar”. It also announces a ban on the sale of energy drinks to under-16s, alongside a slew of other policies aimed at improving public health.

The government has been consulting on the proposals for months and Hancock’s junior minister, Seema Kennedy, had been expected to launch the green paper within days. However, in his latest headline-grabbing campaign pledge, Johnson said he would carry out a review of whether “stealth sin taxes” were successful in changing behaviour and whether they disproportionately affected poorer consumers.

“The recent proposal for a tax on milkshakes seems to me to clobber those who can least afford it. If we want people to lose weight and live healthier lifestyles, we should encourage people to walk, cycle and generally do more exercise. Rather than just taxing people more, we should look at how effective the so-called ‘sin taxes’ really are, and if they actually change behaviour,” Johnson said.

“Once we leave the EU on 31 October, we will have a historic opportunity to change the way politics is done in this country. A good way to start would be basing tax policy on clear evidence.”

Q&A Why is sugar bad for you? Show Eating too much sugar contributes to people having too many calories during the day, which can lead to weight gain. Being overweight increases the risk of health problems such as heart diseases, type 2 diabetes, and some forms of cancer. Sugar is also one of the main causes of tooth decay. The NHS advises that most adults and children in the UK eat too much of a type of sugar called 'free sugars'. These are the sugars added to food and drinks, found in biscuits, chocolate, breakfast cereals and fizzy drinks. But they are also found naturally in honey and unsweetened fruit juices. The UK government’s recommendation is that these 'free sugars' should not make up more than 5% of the calories you have every day. That is still quite a lot of sugar - it equates to seven sugar cubes worth for an adult. But bear in mind that one can of a fizzy drink can include the equivalent of 9 cubes of sugar. Children under 4 should avoid all sugar-sweetened drinks and food with added 'free sugars' in it. Martin Belam



His promise appeared to be designed to enthuse Tory members concerned about unnecessary restrictions imposed by the “nanny state”. But it also seemed to cut across months of painstaking work carried out by Hancock’s department – and Johnson’s own decision to impose a 10p levy on sugary drinks at City Hall when he was the mayor of London.

Hancock quit the leadership race after coming sixth in the first ballot and unexpectedly threw his weight behind Johnson. Allies claimed at the time he would be able to hold the frontrunner to his promise to govern as a One Nation Conservative from the centre ground of politics.

The health secretary praised the sugar levy in a recent speech, saying it had “removed the equivalent of 90m kg of sugar since it was introduced in 2016, proving that population-wide measures work and are necessary, alongside promoting healthier behaviours and empowering individuals to make better choices.”

Johnson is not categorically ruling out extending the tax. He said he would first conduct a “comprehensive review” into the effectiveness of such measures and would halt the rollout of any new “sin taxes” until it had been completed.

Health campaigners expressed disappointment. Shirley Cramer, the chief executive of the Royal Society for Public Health, said: “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. Nearly half of the soft drinks market has reduced the sugar in their products to avoid charges. 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.

A spokesperson for Hancock’s department pointed to the success of the sugar levy and said: “Our policies on obesity and public health have always been guided by evidence and will continue to be in the future.”