The National Union of Teachers (NUT) has voted against balloting members on a boycott of primary school tests, known as Sats, in England.
Delegates at the union's conference rejected a resolution to ballot members over a protest against primary school tests for the academic year 2017-18.
They also agreed not to "support and promote a parent boycott" of the 2017 national curriculum tests (Sats).
The vote came despite two sessions of argument in favour of action.
A vote on a separate motion earlier in the conference backed moves to consider a ballot of membership over a boycott of tests in 2017-18 at a later stage, if there was enough support.
Opposing the motion, Sasha Elliott, a teacher from east London, said: "I've been coming to conference for over a decade, I've made speeches about the wickedness of Sats, ending Sats has to remain one of our union's highest priorities.
"But I'd like to think that we've learned from our past efforts to end these Sats, we have to admit we've been unsuccessful.
"This motion presents us with some serious problems... It's a waste of a precious ballot."
Speaking in favour of action, Nottinghamshire teacher Gareth Jones said: "It feels to me that we've been preparing for action since I've been in the union in 2007.
"If we haven't prepared enough for this action by now, frankly, we never will.
"During this time we've been preparing, thousands of children have had their education destroyed by the cruel tyranny of these assessments."
The debate began on Sunday but was delayed twice due to strict conference timing rules.
Conversely, at its annual conference before Easter, the Association of Teachers and Lecturers voted in favour of a boycott of Sats tests in the summer of 2018.
NUT's rejection of a boycott comes just weeks after the government announced plans to scrap national curriculum tests for seven-year-olds.
The Department for Education is currently consulting on a range of proposals regarding primary school testing.
It maintains that parents have a right to expect testing in schools to show whether their children are leaving primary school with the right skills in maths and literacy.
In 2016, the first set of Sats on the new national curriculum were taken by pupils in England's schools.
For Year 6 pupils, those at the end of their primary education, the pass rate fell from 80% in 2015 to 53%.