School heads are threatening to withdraw from an agreement on teachers' workload unless the current cash crisis is addressed by ministers.

The National Association of Head Teachers has threatened to withdraw from the new system of classroom assistants unless an additional £2.5 billion is found.Union chief David Hart said his members would be forced to withdraw from the scheme unless they are given the money to implement it.

The union is also set to call for the scrapping of tests for seven year-olds.

Speaking to the union's conference, education minister Stephen Twigg insisted councils were still holding back large sums meant for schools.

He also said that schools should be allowed to run a budget deficit in order to overcome any cash shortfall.