Britain's biggest teaching union has voted to continue its boycott of the tests and to back it up with strike action if necessary.
The National Union of Teachers could face court action over its stance. The decision was taken at the union's annual conference in Scarborough, north Yorkshire, which ends today.
The boycott was declared legal last year on the basis that it was over teachers' workloads, but the NUT has also declared its opposition to the tests on educational grounds. The other two main teaching unions, the Association of Teachers and Lecturers (ATL) and the National Association of Schoolmasters/Union of Women Teachers (NASUWT), have withdrawn their boycotts of tests in the light of a government commitment to slim them down.
Yesterday, NUT members accused their sister organisations of becoming too close to ministers - Baroness Blatch, the Minister of State for Education, will speak at both the NASUWT and the ATL conferences this week.
It is rumoured that she may announce the use of external markers for the tests, a move which would be opposed by the NUT on the grounds that it would undermine teachers' professional standing.
The NUT is also likely to hold local one-day strikes this year over redundancies, class sizes and new contracts for members in sixth- form colleges.
Marian Darke, last year's president of the union, told the conference that to carry out the tests would be professionally and morally indefensible.
'Our position is absolutely clear,' she said. 'We will not carry out tests that are educationally unsound. We are against additional workload, but we are also against unnecessary workload.'
Doug McAvoy, the general secretary, said the use of tests for league tables was another factor which reinforced the union's implacable opposition to them.
If its position led to the NUT being taken to court, it would mount a strong defence, he added.
'We will argue that there is still unnecessary and additional workload, but we have never sought to conceal our opposition to the principle of the tests being used for league tables or our opposition to the educational unsoundness of them,' he said.
John Patten, Secretary of State for Education, told BBC radio yesterday that he was disappointed by the NUT's decision to take industrial action.
He said: 'I am very saddened, and I think parents will be frankly bewildered that just when standards are beginning to improve. . . we are suddenly back in the middle of the Sixties. We could have one- day strikes right in the middle of the A-levels - that can't be right.'
Doug McAvoy, general secretary of the NUT, faces a leadership challenge from his left-wing deputy. Mary Hufford, who has worked with Mr McAvoy for the past five years, will stand against him when he comes up for re-election in June. She said at the weekend that his pounds 67,000 salary was excessive and that recent trips to America and Australia were a distraction from the real job of running the union.