One in three low-income families are missing out on government-funded vouchers for free fruit, vegetables and milk that are intended to improve the diets of pregnant women, babies and children.

Out of 402,384 people on low incomes in England who are eligible to receive Healthy Start vouchers, 135,671 (33.7%) are not doing so, figures show.

Women who are at least 10 weeks pregnant or who have a child under the age of four can get one or more vouchers worth £3.10 a week to spend on nutritious foods at participating stores. The vouchers are available to any woman who receives benefits such as income support or jobseeker’s allowance, or who lives in a home where someone does.

The vouchers were brought in UK-wide in 2006 by Labour in an effort to improve the diets of poorer families and narrow the nutritional gap between rich and poor.

Specialist children’s doctors and public health experts have praised the scheme for playing a key role in tackling obesity and ensuring low-income families can access healthy foods that some may otherwise struggle to afford.

The Labour MP Frank Field, a former welfare minister and veteran anti-poverty campaigner, who obtained the take-up figures through parliamentary questions, said: “Healthy Start vouchers have quietly developed into a key weapon in our armoury to defeat malnutrition. They radically improve poorer families’ diets by increasing their consumption of fruit and vegetables as well as vitamin intake, while simultaneously reducing the pressure on their weekly budgets.

“As things stand, too many of the most disadvantaged families remain exposed to malnutrition because they are not registered for the scheme.”

Frank Field called Healthy Start a ‘key weapon in our armoury to defeat malnutrition’. Photograph: Christopher Thomond/The Guardian

Prof Russell Viner, president of the Royal College of Paediatrics and Child Health, said the low take-up was worrying given rising obesity.

“Eating well early in life is key to ensuring good eating habits last into adulthood,” he said. “We know that children living in the UK’s poorest households are more likely to be overweight and have poor nutrition, which is why this scheme is vital.

“The low uptake is particularly concerning as this week a commitment to deliver the Healthy Start vouchers appears to have been dropped as a priority from the Department for Health and Social Care’s business plan.”

Failure to register could be linked to not knowing the vouchers exist, the system’s complexity, and stigma of exchanging them for food, he said.

Across England, take-up of the vouchers is lowest in Windsor and Maidenhead, Theresa May’s constituency, at just 47%. Other areas with low levels of take-up include Wokingham and Buckinghamshire, where take-up is 52%.

Hartlepool has the highest take-up rate, at 84%, followed by Newcastle and Halton, Cheshire, where 82% of those eligible are registered for them.

“My plea to the prime minister is to take all necessary steps towards securing full take-up, perhaps by initiating a policy of automatic registration which overcomes issues around stigma and complexity,” Field said.

Research published last week by academics at Bristol and Manchester universities found that the vouchers had produced “significant nutritional improvements” in poorer families’ diets. It found that such families now bought 15% more fruit and vegetables then before.

A spokesperson for the Department of Health and Social Care said: “The Healthy Start scheme provides vital assistance to help children from lower income families access free fruit, vegetables, milk and vitamins.

“While participating is a personal choice for eligible families, we publicise it widely to encourage uptake. It is also promoted at a local level through midwives and healthcare professionals, and we are exploring how digital technology could make it easier to receive Healthy Start benefits in the future.”