Chris Thatcher, president of the National Association for Head Teachers, made the call after many schools - including his own primary in Coventry - were billed for thousands of pounds for Internet access.

BT’s tariff gives schools unlimited access to the Net for pound;790 a year, but only for calls between 8am and 6pm Monday to Friday during term times. Many of the three-quarters of schools that access the Net do so on BT even if they use other Net providers.

Two of these, RM and Edex, have found some schools are receiving very high bills. The cause is thought to be Windows NT networks using browsers and email programs that are not shut down correctly and trigger calls throughout the night.

Some schools only became aware of the problem, which first came to light at the start of this year, when the bills started rolling in. One Oxford secondary received a bill for pound;3,489.29 and a primary in Cheshire was perplexed by a 76-page bill for pound;1,125.

Tim Clark, RM’s Internet marketing director, said it has been monitoring call usage and has notified more than 50 schools that have a problem.

Mr Thatcher said BT did not recognise that schools stay open after 6pm and during holidays and could not afford big phone bills. “Schools can’t be seen by BT as a source of income,” he said.

Mr Clark said RM was advising schools on how to stop unnecessary calls being made, but he supported calls for BT to change its policy. “They should stop this restriction - it can’t cost them much more to do so,” he said A Department of Education and Employment spokesperson said the Government realised schools wanted to use the Net more, particularly as links with communities developed. A proposal on access for schools charges was expected after Online went to press.

Adrian Carey, education services director for Edex, started receiving complaints from schools at the rate of 30 to 40 a day. He said: “They were livid - it was a nightmare.”

He believes it is almost impossible to prevent the problem. He agreed a change in BT policy would ease the situation, but said it highlighted the need for a national strategy to give schools permanent broadband Internet links rather than dial-up links.

If Britain had devised a broadband solution, he said schools would have permanent high-speed connectivity without the problems associated with dial-up access methods such as ISDN.

Edex put plans for a national education network to former schools minister Charles Clarke, but has not yet spoken to Michael Wills, the new learning and technology minister, on the issue.

A BT spokesperson said no schools had complained about excessive ISDN bills, but if there were cases the company would try to devise a “sensible solution”.

Meanwhile, BT has come under fire from AOL, the world’s largest Internet service provider. Andreas Schmidt, AOL European president, accused BT of “ripping off customers” and warned that Britain would be disadvantaged unless the cost of going online fell.

Gordon Brown, the Chancellor, last month said he would encourage more competition between phone companies to drive down the cost of Net access.