Labour has called on Matt Hancock to “consider his position” as health secretary after he backed a review into ‘sin taxes’ proposed by Tory leadership hopeful Boris Johnson.

Hancock – who had been expected to extend the government’s tax on sugary drinks to include milkshakes – hailed Johnson as shining example of getting “fit and svelte” without the need of the “nanny state”. 

“I strongly support having an evidence-based review into how these taxes are working,” he told Sky News. “Of course there’s the tax, but there’s more ways we can make sure we tackle obesity”. 

The health secretary had previously celebrated taxes on sugar, telling the all-party parliamentary group on obesity in January: “The sugar levy has removed the equivalent of 90 million kilograms 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.” 

Having been eliminated from the race to replace Theresa May in Number 10, Hancock has since given his backing to Johnson, the bookies favourite in the leadership contest. 

Shadow health secretary Jonathan Ashworth branded Hancock’s decision to back Johnson’s “incredible”, tweeting: “We now have a health secretary acting as a cheerleader to Johnson, who is not only ignoring [the] advice of his own department, all medical opinion, but even the advice he himself gave Cabinet just this week.” 

The Tory frontbencher is “simply not fit to be health secretary” and should “consider his position”, Ashworth added. 

The Mirror reported that deputy Labour leader Tom Watson had written to Hancock telling him he must leave the Cabinet if the sugar tax is scrapped. 

“For a prime minister of a nation in the grip of an obesity crisis to scrap the sugar tax would not just be gravely irresponsible, it would be negligent,” he wrote. 

“It would be a gift to companies like Coca Cola and Nestle and a terrible blow to the children and adults who are overweight or obese, whom the government has a responsibility to support, and our National Health Service which treats them.” 

The row comes after Johnson pledged a review into the effectiveness of sin taxes – such as the government’s levy on sugary drinks – and to halt the rollout of any similar levies in the meantime. 

These taxes “clobber those who can least afford it”, the wannabe PM said.

The announcement of the proposal came on the same day Cancer Research UK revealed that obesity now causes more cases of bowel, kidney, ovarian and liver cancers than smoking.

Johnson’s leadership rival – and Hancock’s predecessor as health secretary – Jeremy Hunt criticised the policy, saying: “We have an obesity epidemic.

“We have the second highest number of obese young people anywhere in Europe and we do need to have a solution to this.” 

The people who want to scrap sin taxes need to unveil their alternative plans “because it’s terrible for the young people involved”, Hunt added. 

Meanwhile, the PM’s official spokesman defended the tax on sugary drinks, saying: 

“Over half of the drinks in scope of the levy have been reformulated and that is the equivalent of removing 45m kilograms of sugar every year.

Asked if the sugar tax had been effective, they replied: “That statistic illustrates that it has been.”