John Patten, former Secretary of State for Education, proposed the plan last month as a way of solving the dispute over teachers' workload.
But the National Association of Head Teachers said the idea of external markers for national tests for 11- and 14-year-olds and extra teachers to cover for those administering tests for 7-year-olds caused it 'grave concern'.
The Secondary Heads Association has already announced its opposition to external markers. Though two of the other four teacher unions support external marking, another has strong reservations and the one continuing the boycott, the National Union of Teachers, does not.
The heads' decision means that Gillian Shephard, Mr Patten's successor, may have to look for other ways of countering teachers' contention that the tests involve an unreasonable amount of work.
Heads said the Government proposals would devalue teacher assessment which was a fairer way of judging pupils' performance.
They also complained that it would cost too much - about pounds 12m once savings from monitoring teacher assessment have been made, according to the Department for Education.
David Hart, the association's general secretary, said both the idea of placing a statutory commitment on teachers to do the tests and that of changing the tests to lighten workload had apparently been abandoned.
'It is difficult to understand how teachers who find the marking of 30 sets of scripts excessive would not find the marking of 300 sets of scripts excessive because they were being paid for it.'
Despite reports that the Government is considering changing the law to compel teachers to mark national tests, the Department for Education said ministers had 'made no such commitment'. A spokesman said heads had a legal duty to implement tests and teachers were under a contractual duty to carry out their instructions.Reuse content