The Education Secretary, David Blunkett, announced the ban last week as part of a strategy for improving primary maths which was devised by a government task force.

But the General Secretary of the National Association of Head Teachers, David Hart, says the task force - headed by Professor David Reynolds of Newcastle University - did not recommend a calculator ban.

In a letter to Mr Blunkett, Mr Hart writes: "Anybody with an ounce of common sense knows that calculators should never be used to the detriment of acquiring basic skills, but they can have a purpose, even for children up to age eight.

David Hart: "I do not see how any attempted ban could be remotely enforceable"

"Teachers should be allowed to use their professional judgement in this area. To ban calculators is to interfere with their professional discretion and does not enhance their image."

The use of calculators is covered in National Curriculum orders - the regulations which specify what children must be taught.

These are due to be reviewed by the Qualifications and Curriculum Authority as part of an overhaul of the curriculum which is due to come into force in the year 2000.

Injunctions

But Mr Hart said the government had no power to stop teachers showing children how to use calculators.

"I do not understand why the Secretary of State chose to convert an expression of opinion on the use of calculators, which many teachers will share, into a statement that they are going to be banned.

"Does he imagine that the courts are going to come down on schools and issue injunctions?"

Professor Reynolds said: "The report of the task force did not recommend a ban on calculators.

"The view of the task force was that the use of calculators was an issue for the Qualifications and Curriculum Authority's review of the National Curriculum."