A row has erupted over whether flavoured milk shakes can be served in schools - with dairy bosses accusing the School Food Trust of misrepresenting the law.

The dispute centres on guidance issued to schools by the Trust explaining the regulation introduced last year that restricts the food and drink that can be served at school dinners.

This guidance states that milk beverages containing cocoa or added fruit or vegetable juice are permitted.

However, there is no mention in the list of milk shakes made with added artificial flavours, which suggests they are banned.

Trade body Dairy UK said it believed the Trust's guidance did not precisely reflect the regulation itself.

Although the UK government could restrict sugar content, it couldn't ban milk shakes made with artificial flavours because EU laws covering permitted additives prevented this.

Dairy UK technical director Ed Komorowski said it was not clear whether the Trust had deliberately omitted milk shakes from the list of permitted drinks or whether it was a genuine mistake.

"The Trust may have felt it was frustrated by the need to comply with the EU's permitted additives rules and, having realised that, may have attempted to apply further restrictions through its guidance to schools," he said. "Or it may simply have been a misinterpretation of the regulation."

The School Food Trust insisted it had not got its facts wrong. Judy Hargadon, chief executive, said confusion had come about because in its initial guidance to schools - published before the wording of the regulation was finalised - it had implied flavoured milks would be permitted.

But since the law had come into effect last September, it had amended its guidance to make it consistent with the law. "We will work with the dairy industry to agree the best way forward," she added.

The matter is of urgency because from September new rules come into force covering the food and drink served in schools throughout the whole day, not just at lunch.

The Trust is also to produce a voluntary code of conduct to encourage schools not to sell products containing additives.