A growing number of children are developing vitamin D deficiency, which can cause rickets and seizures, ministers have warned.
Dozens of cases of rickets, a bone disease thought to have been wiped out in the 1950s, have been diagnosed.
The disease is making a resurgence, a spokesman for the Department of Health has warned.
Studies suggest between seven and eight children per 100,000 have vitamin D deficiency. But just 15 minutes of summer sunlight a day is enough for the body to manufacture the required amounts.
Families who qualify for the Healthy Start scheme can obtain free supplements. Dawn Primarolo, the health minister, said: "We particularly encourage women who are pregnant or breastfeeding to take vitamin D, to protect the health and well-being of their baby."