Members of England's largest teaching union the NUT have been urged to stand firm with head teachers and vote for a boycott of Sats tests.
The call came from the head of primary school head teachers' union the NAHT which has joined the NUT against the national tests in English and maths.
Mick Brookes got a standing ovation at the NUT's conference in Liverpool as he said it was a "unique moment".
Sats are due to be taken by 11-year-olds in England in a few weeks' time.
The other big classroom teachers' union in England, the NASUWT - also meeting this weekend - is opposed to the boycott.
The government has appealed to the unions not to disrupt the tests but to work with it on changes.
The Conservatives and the Liberal Democrats are against a boycott but say they would reform the system.
England is the only part of the UK which has Sats tests.
At last year's conferences, the NUT and the NAHT (National Association of Head Teachers) voted to ballot on boycotting the Sats if efforts to persuade the government to reform them did not work.
While all members of the NAHT are being balloted over the threatened action, in the NUT only those who are heads or deputies are. They oversee the running of Sats tests in schools.
Mr Brookes told the conference: "The journey we have been on has been a great one.
"I am really proud we are standing up for what matters, with the NAHT and the NUT working together."
He said there were head teachers who were behind the action, about 5 or 6% opposed on principle and "most of all there are those who are afraid".
He said:" To those I say 'That is the very reason to vote yes'".
He told delegates there was "no doubt this was a lawful and valid dispute".
After Mr Brookes's speech, the general secretary of the NUT Christine Blower told delegates: "Together we are better. We can get rid of Sats."
The government has urged teachers not to take industrial action saying a boycott of Sats would be a breach of their statutory duties.
Schools minister Vernon Coaker said: "We don't think going ahead with a boycott would be in children's best interests and we urge heads and teachers to use their professional judgment and keep talking to government about the future of testing.
"We have made clear that we are committed to improving the assessment and accountability system to ensure it is fair to schools and teachers."
A spokesman for the Department for Children, Schools and Families said the issue is accountability.
The government is introducing the School Report Card to give a wider view of schools' achievement, he said.
"It is a move away from the narrow measure of traditional league tables to give a broader picture of how schools are performing for every child."
The question is whether strength of opposition at the conferences will be translated into a vote for the boycott by the time the ballots close on 16 April.
Both unions have carried out what they call "indicative ballots" of members and say a majority said they would back action.
For the NUT, 76% said they were "prepared to take action to boycott the tests should it be necessary".
Speaking after his address, Mr Brookes said two-thirds of the NAHT members who took part felt the same.
Mr Brookes said together, the two unions represented staff in a majority of England's 17,000 primary schools and that returns from the indicative ballot suggested "6,000 to 7,000" would be hit by any industrial action.
Ms Blower said having "2,000 to 3,000" schools not taking the tests would "frustrate them", because this would de-value this year's league tables.
Opposition to the league tables is one of the main reasons both unions oppose the Sats as they say they "humiliate" and pressurise schools and teachers.
The unions also argue tests are bad for children's education because teachers feel they have to "teach to the test" and concentrate on maths and English at the expense of other subjects.
The league tables are not published by the government but are drawn up from the Sats results. Newspapers and media organisations compile the tables.
Any action would be open to legal challenge from the government because of the statutory duty on heads to administer the tests.
Leaders of the NUT are also urging members to back calls for industrial action to oppose any budget cuts in schools in England.
They will debate an emergency motion on Monday which would commit the union to campaigning against cuts with unions representing other public sector workers.
Schools budgets are to rise by 0.7% in real terms in the next three years.
But this will be funded by "efficiency savings" of £650 million in procurement for schools and £50 million by schools saving energy.