When asked at a Tory hustings in Exeter last week whether the sugar tax should be extended to milkshakes, Johnson said he would be reluctant to impose new legislation. 

“There is an obesity problem in this country. Something we all deal with and wrestle with,” he said. “I am reluctant to impose new taxes that impose disproportionately those on low income. 

“If you want kids to lose weight, make the streets safe, like we did in London. Encourage kids to walk and cycle to school. It’s calories in, calories out.”

Johnson is competing against Jeremy Hunt for the positions of Tory leader and prime minister.