Investment in schools is being threatened by Britain's largest teaching union, the NUT, the government said yesterday.

Ministers have set a deadline of next Wednesday for agreement on reforms tied to the money, which schools are scheduled to receive over the next three years.

The National Union of Teachers, which has 220,000 members, is refusing to agree to plans to allow them a day off a fortnight for marking and preparation because it depends on their allowing unqualified assistants to take over certain lessons.

Schools minister David Miliband yesterday pledged to continue with plans to use classroom assistants despite opposition from unions.