The majority of primary heads have backed a proposed boycott of next year's key stage 2 Sats, according to results of an official consultation.
Teaching union the NAHT says a survey of around 22,000 heads, assistant heads and deputies revealed "strong support" for action.
The union is now confident it would win a full ballot of members if the Government does not give in to its demands to overhaul the testing regime.
The positive response to the consultation comes despite some heads' concerns over the legal implications of refusing to administer the tests next May. The NAHT has mounted a joint campaign against Sats with fellow union the NUT.
Although the NAHT is yet to release detailed findings, Mick Brookes, the union's general secretary, said it had "strengthened its resolve" to push for an end to national tests and the introduction of a new teacher-led assessment for 11-year-olds.
Ed Balls, Schools Secretary, told The TES this week that his department was "always willing to change" and was "taking teachers and headteachers seriously".
In one recent concession, he announced that the results of teacher assessments would be published alongside Sats results from 2011, although this was criticised by Mr Brookes for not going far enough.
A final decision on whether to carry out a full ballot of members will be made with the NUT, which is expecting the result of its own indicative members ballot next week.
The views of headteachers are seen as key as they will make the final decision on whether to take their pupils out of the tests.
Steve Iredale, chair of the NAHT's curriculum and assessment committee, said turnout for the consultation was "a lot higher than normal for a trade union", although it is understood that the response rate was below 50 per cent.
"The majority of responses have also indicated strong support for action," he said. "It would be enough to go towards a full ballot.
"I feel that if there was a ballot and we were pushed to the brink, that ballot would be a success."
The timing of any boycott would prove embarrassing to the Government in the run-up to the general election, which has been widely predicted for May.
But the union has also stressed it hopes the outcome of the consultation will be a useful negotiating tool in reaching an agreement with Government on an alternative assessment regime.
Heads contacted by The TES showed mixed support for a boycott. Dr Neil Suggett, head of Hayes Park Primary in Hillingdon, said that the tests cast a "long shadow" over school life, and said he would be happy to take part in a potential boycott.
"I think the tests skew the primary curriculum in a way that hinders the development of children enormously," he said.
"I have been teaching for 40 years and I remember what it was like before the Sats when the curriculum was not narrowed by preparing for the tests."
But David Deane, head of St Thomas of Canterbury Primary School in Salford, which won the title of most improved school in England this year, said: "I don't support the boycott because I don't think that to test at the end of four years of education is unreasonable."
The results of the consultation come as the NAHT released its own detailed vision for the future of assessment at KS2 today.
It calls for a move towards internal teacher-led assessment, with staff reporting pupils' progress directly back to parents.
A combination of chartered assessors, school improvement partners and Ofsted would ensure that teachers were carrying out the assessment properly.
"Rotational sampling", where 30 per cent of pupils nationwide would sit external tests in English, maths and science would allow the Government to gauge the progress of the education system as a whole, but individual schools would not be named in the results.
The charter also calls for the Government to develop its idea of the School Report Card, but using a wider set of measures to judge schools.
Mick Brookes said that the union's vision would result in "more accurate information" being given to schools about Year 7 pupils. He also said it would provide a better educational experience for children in Year 6 and accountability systems that are based on the actual achievements of children in the primary sector, rather than the proxy of test results."