Teachers are expected to vote on Monday on whether or not boycott key primary school tests amid concerns over the pressure it puts on pupils.
Delegates at the National Union of Teachers (NUT) annual conference in Cardiff will look to pass a resolution to "support and promote a parent boycott" of the 2017 national curriculum tests for primary school children, known as Sats.
The motion was listed for consideration on Sunday, but conference delegates, all of whom spoke in favour of action, ran out of time.
Kevin Courtney, NUT general secretary, said: "What I heard was teachers giving you their professional assessment of what the assessment system is doing in schools. They want a change.
"Many of them also said teachers aren't against testing, they were speaking in favour of a positive alternative.
"They were saying a boycott would be a completely defensible, moral position to take to stop this happening to our children, and they're right to think that."
Any vote in favour of the boycott would mirror that of the Association of Teachers and Lecturers, where members at its annual conference in Liverpool this month voted for similar action. The unions are to merge later this year.
The Department for Education said: "We want a long-term, stable and proportionate system for primary assessment that measures the progress that children make throughout their time at primary school fairly and accurately, one that recognises teachers' professionalism in assessing their pupils, and which does not impose a disproportionate burden.
"We have worked with the teaching profession on how best to establish this and we are currently consulting on a number of proposals."