The suggestion that the NUT and NAHT might refuse to administer next year's primary tests has upset the Government - and the other unions, which feel the move will make ministers less likely to co-operate on replacing the hated Sats.
The debacle of last year's key stage tests created stress and anger for thousands of teachers. But the shambles of late results finally led to a cause for almost universal celebration: the abolition of those same tests for 14-year-olds.
But while secondary teachers and heads cheered, primary school staff were left frustrated and exasperated. Why, they demanded, weren't the equally discredited tests for 11-year-olds being scrapped too?
Their deep disappointment, left to ferment over the past five months, led to last week's dramatic announcement from the National Association of Head Teachers (NAHT) and the NUT that both unions were considering boycotting the tests for primary pupils at both seven and 11 from next year. It is widely expected that members of both unions, who will be given the final say at their annual conferences over the next month, will say yes to the proposals.
But questions have already been raised over whether a boycott is the best way for unions to get the tests abolished. Does the move turn what was a winnable argument into a game of political brinkmanship where neither the unions nor the Government will want to blink first? And because administering the tests is a legal duty, are teachers running the risk of disciplinary action for refusing to take part?
Christine Blower, the NUT's acting general secretary, said ministers should see the unions' action as a positive move - something they can use to justify action on primary school tests. "We are perfectly happy to negotiate to avoid the need for this action," she said. "That is why we've given so much notice of our intentions. It would have been reckless to consider a change this year without giving significant thought to how our members will assess pupils, but we have done that. There is plenty of time for a reasonable meeting of minds about what needs to take place."
Such optimism about a happy resolution, however, already seems misplaced.
The proposed boycott was met with an uncharacteristically strong response from the Department for Children, Schools and Families, which criticised it for being a retrograde step that removed parents' basic rights to know how their children were performing in school. This provoked a retort from the NUT that the Government was being "deliberately and utterly disingenuous".
Mick Brookes, general secretary of the NAHT, said ministers had already been told that information about pupils' progress during key stage 2 would be passed on to parents based on teacher assessment.
"We would point out that in 2008 many parents and children received no information about their child's Sats result and many others received information that was flawed by the appalling standard of marking," said Mr Brookes.
"We are amazed that a socialist government continues to allow the publication of raw data league tables which are used on an annual basis to demean the work of colleagues in the most challenging schools.
"To suggest that colleagues wish to hide information from parents is deliberately misleading, insulting and outrageous."
Mr Brookes also talks of wanting a negotiated solution. But he wants that to come with an acknowledgement from ministers that they are irresponsible in their desire to continue with tests - not a likely scenario, especially as next year's Sats - in May - will be close to the expected date of the next general election.
That leaves the question of whether schools will carry out any edict to boycott and what the fallout will be.
The legal situation is unclear. It is the statutory duty of headteachers to administer the tests, so failing to do so risks leaving them in a vulnerable position.
Mr Brookes said his union had taken legal advice and is confident that it can justify industrial action on the grounds of a trade dispute.
But if that fails to hold water, teachers could find themselves facing disciplinary action. Heads, especially, are in a difficult position as they may support the boycott but be managing teachers who are members of unions that do not.
Phil Revell, the chief executive of the National Governors' Association, said any head who pressurises a teacher into a boycott against their wishes will face disciplinary action. "We really regret that headteachers are going down the road of a boycott," said Mr Revell. "They are placing their governing bodies and their teachers in a very difficult position."
He expects disciplinary action will be an option for governors however the boycott is implemented, but admits that many governing bodies will want to avoid that sort of confrontation.
"Taking disciplinary action against heads may not be seen as good industrial relations," he said. "They will have to bear in mind their future relationship and the impact on the school."
The General Teaching Council for England has not so far dealt with a case of a teacher refusing to administer Sats, but said it would investigate any case referred to it if a boycott went ahead.
The other teaching unions have been unusually outspoken in condemning the proposed boycott. Mary Bousted, general secretary of the Association of Teachers and Lecturers (ATL), said their action was shifting focus attention away from the single-level tests that could replace Sats. A pilot of these tests - which mean pupils sitting exams when their teachers feel they have reached the appropriate ability level - is being conducted.
The Government has not made any firm commitment that single-level tests will replace Sats, but appears keen on them if it can make them work.
"The key stage 2 tests are on their way out and our concern is what replaces them," said Dr Bousted. "Single-level tests would be disastrous because they would take the testing pressure that exists in Year 6 and impose it on Years 4 and 5 as well.
"A boycott is a distraction from the real issue. It will also make the Government less rather than more likely to co-operate with what the unions and teachers want."
Martin Johnson, the ATL's deputy general secretary, said the NUT, which has not joined the social partnership with the Government, had the excuse of being "out of the loop" in reaching its decision. On the other hand, he says, the NAHT "should be aware, but doesn't get it".
The ATL supports the view that key stage tests should be abolished and replaced by both teacher assessment and a national sampling scheme to monitor progress in English, maths and science.
The NASUWT, the second biggest teacher union after the NUT, wants performance tables abolished, but is concerned that greater use of assessment could lead to "excessive burdens" being placed on teachers.
In Wales, where Sats have been scrapped, teachers were already "buckling under the weight of bureaucratically burdensome teacher assessment and tests in every year, instead of just at the key stages," said Chris Keates, the union's general secretary.
John Dunford, general secretary of the Association of School and College Leaders, the other headteachers' union which represents the secondary rather than primary sector, also criticised the boycott.
"Secondary school leaders are very clear that there has to be a substantial external component to key stage 2 assessment because it forms the baseline for secondary school accountability," he said.
"We argued against key stage 3 tests for many years and in the end won the argument. Changes to key stage 2 should come from a similar debate.
"I would question if a boycott might make change less likely as ministers have never been known to give way easily to such tactics."
How a boycott could backfire
Ministers and government officials have been left "annoyed and mystified" by the plans for a boycott of primary school national tests, according to sources.
The move risks the National Association of Head Teachers being sidelined from future discussions and its influence being diminished inside the Department for Children, Schools and Families, it has been claimed.
"Having debates with the Government is how you influence policy," said one source. "It is not changed by threatening ministers with this kind of thing."
The NAHT is part of the "social partnership", which includes the teaching unions and the Government. Ministers and union officials meet regularly to discuss issues of school workload.
The NUT is not involved in the partnership, so it already has a more limited relationship with the Government.