GPs, midwives and health service officials have been told to ensure pregnant women and those with young children in low-income families know of their right to free vitamin D supplements.

Takeup of vitamins under the NHS's Healthy Start scheme is low, according to Sally Davies, the UK government's senior medical adviser, and chief medical officers in Scotland, Wales and Northern Ireland.

Up to a quarter of the population are at risk of vitamin D deficiency, according to national nutrition surveys. This has led to concerns about the health of bones, particularly in older people and young children.

Those at risk include people who have low or no exposure to the sun, for example those who cover their skin for cultural reasons, who are housebound or confined indoors for long periods.

People who have darker skin may also need supplements because their bodies may not be able to make as much vitamin D.

The medical officers say the NHS must ensure those who need supplementation are offered it, in the form of tablets for adults and drops for children.

Women qualify for Healthy Start from the 10th week of pregnancy or if they have a child under four years old, or if they or their family receive income support, jobseeker's allowance, employment and support allowance or child tax credit. Pregnant women under 18 also qualify, even if they do not get any of those benefits or tax credits.

The letter adds: "NHS organisations can choose to sell the vitamins or supply them free of charge to those who are not eligible for Healthy Start, and we encourage this. Alternatively, vitamin D supplements are available for purchase or can be prescribed for those who are not eligible for the scheme."