Downing Street is proposing a "fat tax" on junk food in order to curb rising rates of obesity.

The prime minister’s strategy unit says the move is needed because heart disease has overtaken cancer as Britain’s biggest killer and more young people have started developing diabetes.

The strategy unit's paper, seen by the Times, says: "Some of the risks associated with (obesity), such as cardiovascular disease, can be mitigated to some extent with drugs, and NHS expenditure on these is rising rapidly.

"However, the main drivers — poor diet and a sedentary lifestyle — are largely outside the direct influence of the NHS.”