The National Union of Teachers in England and Wales will stage their first national strike in 21 years on Thursday, forcing the closure or partial closure of thousands of schools.

Gordon Brown said the teachers' strike was "very regrettable", telling MPs during Prime Minister's Question Time that he hoped teachers would "reconsider" any further industrial action.

"I think it is very regrettable that the teachers are going on strike and the reason is that even the chairman of the pay review body has said this is an independent award, independently adjudicated, one that the teachers should be prepared to accept.

"I hope that after reflection the teachers will reconsider the action that they are going to take in the future on this matter."

The British Chambers of Commerce warned the strike could cost businesses up to £68 million in lost working hours because parents will have to take time off work or pay for extra childcare.

Around a third of schools will be hit by the strike, with up to 2,000 closing and a similar number partially closing.

Schools minister Jim Knight said the three-year pay deal of 2.45% from September and rises of 2.3% in subsequent years was "fair and reasonable" and would not be improved.

"Parents feel increasingly frustrated and inconvenienced at the loss of education to their children and I share their feelings.

"Given that the average teacher's salary is £34,000 and pay has increased by 19% since 1997 I would urge teachers to teach."

NUT acting general secretary Christine Blower said the strike was a "last resort", adding that teachers had received three years of below inflation pay increases.http://www.teachers.org.uk(NUT)