The government is planning to give all pregnant women a £120 payout to spend on fruit and veg, in a bid to protect their infants from childhood diseases.

Health secretary Alan Johnson will announce the proposal in a speech this week, aimed at highlighting the gap between rich and poor.

However, critics of the scheme, which will be paid to middle-class mothers-to-be as well as poorer households, are expected to suggest the money may just be wasted, as there is nothing to prevent pregnant women spending it on drink, cigarettes, chocolate or even clothes.

The ‘health in pregnancy’ benefit will be made as a one-off payment when a woman is seven months pregnant. The expectant mothers will be expected to take expert advice on the advantages of a healthy, balanced diet, and also to give up smoking and drinking.

The announcement comes as the government’s progress in public health issues comes under closer inspection this week, when former Treasury adviser Sir Derek Wanless is expected to criticise the lack of advancement in tackling obesity.

Johnson’s aides accept that some of the 630,000 women who fall pregnant each year may spend the fresh produce money on other products. Officials at the department of health have decided it would be too complicated and “possibly unfair” to give the money to only the poorest families, so the money is instead likely to become an additional boost in the way child benefit is paid to all parents, regardless of income.

But the proposals, which are expected to cost £80 million annually, are still under scrutiny by the Treasury. Health experts argue the initiative could reduce NHS costs.