Pregnant women are to be offered a £120 "fruit and veg" grant to improve the nutrition of unborn babies in a scheme condemned as a gimmick by opposition parties.

By 2009, all 630,000 women who become pregnant annually in England and Wales will receive the one-off payment at seven months, along with advice on maintaining a balanced diet. The "health in pregnancy" grant is intended to raise birth weights in poor areas and prevent heart disease and other problems developing later in life.

The Conservatives and the Liberal Democrats complained that the £70m-a-year scheme – to be announced this week – would be a waste of taxpayers' money and should be targeted at women most in need of help.

One in 12 babies in England and Wales is born underweight, sometimes because their growth in the womb has been retarded by a lack of nutrients. They are in danger of dying in infancy and at greater risk of long-term heart problems, diabetes, lung conditions and cognitive disorders. Britain's high proportion of teenage pregnancies – the highest in Europe – is partly to blame for the problem.

Announcing the grant at the New Health Network on Wednesday, the Secretary of State for Health, Alan Johnson, will say that, in future, women in the 25th week of their pregnancies will be given advice on healthy eating and the need to give up smoking and drinking.

The Department of Health declined to say whether recipients would be required to give any undertakings or change their behaviour, raising speculation that they could spend the money on anything, including cigarettes or alcohol.

The Conservatives criticised the efficacy of the scheme. "It is vital to improve public health but it can't be done with one-off gimmicks," said Andrew Lansley, the shadow Health Secretary. "This proposal does not appear to result from evaluation of any pilot scheme."

He added: "This Labour Government has presided over growing health inequalities. For the Government now to be talking about enhanced interventions with mothers in deprived areas is deeply hypocritical when the number of midwives hasn't kept pace with the increase in births over the last five years."

Community midwife teams should give mothers more advice, screening and support, he said.

The Liberal Democrats described the grant, first outlined by Gordon Brown in his pre-Budget report in 2006, as "an abject waste" of taxpayers' money. "It smacks of a typical New Labour gimmick that tries to address a serious social issue by throwing money at it," said its health spokesman Norman Lamb.

"The proposal serves to highlight the shameful health inequalities that have embarrassingly widened under this Government. There needs to be far more done to get across the value of healthy diets, especially during pregnancy, but this sort of state handout will likely achieve nothing of any substance."

The Department of Health defended its plan, saying that while women should eat healthily throughout pregnancy, medical advice indicated that nutrition became more important from the 25th week.

"This money is going to be given to women with a package of advice from a GP. It isn't going to be given to them in a brown paper bag," a spokeswoman said.

Tam Fry, the director of the Child Growth Foundation, said that by the time a woman became pregnant, she already needed to be eating well. "Tackling it halfway through the pregnancy is really a bit late, though it is very good that the Government is waking up to the scale of the problem," he told The Observer.