Teachers threatened industrial action against privatising so-called "sin bins" for disruptive pupils.

The numbers of children sent to alternative education centres will rise in the recession, according to the National Union of Teachers.

These pupil referral units (PRUs) provide a valuable service to unruly youngsters, and privatisation will move them further away from mainstream education, the union said.

Rachel Lynch, a special educational needs teacher from Bristol said: "These services should not be used as a dumping ground for children that do not fit into the mainstream.

"Every child has a right to an education and pupil referral units offer a valuable service."

Schools secretary Ed Balls announced details in October of 12 pilot schemes in which private companies will run units for excluded pupils as well as those at risk of exclusion. They included a city farm and a scheme based on Army Cadet Force training.

But at the NUT's annual conference in Cardiff yesterday, delegates vowed to "defend PRUs vigorously from the threat of privatisation".