Most of the alleged offenders are in Northern Ireland, though Hertfordshire charges children Pounds 5 a year for the administration and distribution of their daily third of a pint.
The School Milk Campaign, a national charity run by parents, will condemn the milk charges in its second major report, to be published soon.
Under the 1988 welfare food regulations, children under five should receive free milk in private and state nurseries, primary nursery classes, playgroups and at childminders' homes.
When they reach five, they are charged, but schools can claim a discount from the European Union.
The campaign's 1995 survey, The Sins of Omission, claimed that 12 authorities out of 58 responding to its survey were not giving four-year-olds free milk. They included Belfast, County Armagh and County Antrim.
Stephanie Spiers, co-founder of the campaign, said last week: "Since the 1995 survey many authorities say they are now providing free milk but three areas are charging administration costs. One is charging Pounds 1.85 per term.
"Northern Ireland is one of the poorest areas in Northern Europe. It's an absolute scandal that parents are being charged.
"We estimate that 20 per cent of authorities in the UK are not giving free milk to the under-fives."
Alison Jeynes, an administrator for the welfare food scheme in Belfast, said: "We know we are out of line with the rest of the UK and we are writing to the education and library boards to tell them we are expanding the welfare food scheme to nursery groups."
A letter from Chris Saunders at the Department of Health's welfare food unit to Stephanie Spiers, which has been passed to The TES, said that Hertfordshire's milk charge appeared to be "contrary to the welfare food regulations" and that the department would be writing to the council about it.
The department said it cannot afford to publicise the fact that the milk should be free or to monitor whether nursery classes are providing it. It has, however, written to social services departments, and is in contact with local authority and under-fives associations.
A spokesman for Hertfordshire said: "Our position has been adopted after lengthy discussion and exploration of the legal issues. We are confident we are allowed to make the charge, and we are charging in good faith."
Stephanie Spiers said: "The welfare scheme is in shambles. Hertfordshire is fully reimbursed for milk for the under-fives."
Local management makes it hard to discover how many schools are flouting the regulations.
Essex County Council, for example, encourages its schools to give free milk to the under-fives but cannot give categoric assurances that no schools are charging because they are self-governing.