Headteachers yesterday threatened to boycott a bonus scheme they describe as a "con trick" in a row over a lack of funding for key staff.

Members of the NAHT and SHA unions say more than half the money for performance-related pay will have to come from school bugets.

They claim the will consume more than £1 billion over the next six years, which could have been spent on books and other equipment.

Heads are to be balloted for industrial action after Easter.

John Dunford, the general secretary of the Secondary Heads Association, said: "Heads are not willing to be placed in an impossible position as a result of the government's attempt to run this scheme on the cheap."