Teachers are threatening to boycott the Government's new compulsory tests for four-year-olds, warning the move is "dangerous" to pupils' education.
Ministers announced plans last month for children to sit papers in literacy and numeracy in the first few weeks of the reception year of school. These "baseline" tests, due to be introduced in 2016, could then be used to track pupils progress throughout primary school.
But delegates at the National Union of Teachers (NUT) annual conference in Brighton are expected to raise concerns about the proposals.
A resolution due be debated by the conference says that too much emphasis is being placed on assessment, which condemns children and teachers to simply preparing for tests.
"This in turn creates an unhealthy environment of competition and does little but engender a feeling of failure in children and pressure on parents," it says.
An amendment to the resolution calls on the union to look into the possibility of a "mass campaign of non-compliance" with any policies that they believe "erode children's right to play in the early years".
Speaking ahead of the debate, Hazel Danson - of the NUT's executive - said that there was a "groundswell of feeling" about a move towards more testing for younger children.
"This isn't just the union banging on about something, it's about child development, it's dangerous what would happen," she said.
"If parents understood, I think we would get a lot of parental support and support across the whole of the early education community."
Under the current system, children's education up to age five - including reception year - falls under the early years foundation stage.
Ms Danson added: "I think there would be a groundswell on non-compliance and we would wish to tap into that and encourage that if the Government didn't listen."
She went on to say that there was "significant anger" amongst a small group of members, adding that they would "try and campaign very strongly with parents so that they understand the dangers of what we are being asked to do to their children".
"And there does come a point where you're doing some things that are so wrong and damaging that you can't do it," she added.
In order to boycott the tests, the NUT would need to ballot members for action as part of a trade dispute.
The NUT is also expected to debate a second motion calling on the union to create an "alliance of forces who will build momentum for a campaign to boycott the phonics test".
The test, taken by six-year-olds, is based on ''phonics'' - a system focusing on sounds rather than recognising whole words - which has been hailed by ministers as the best way to teach young children to read.
A Department for Education spokesman said: "We are working with teachers to raise the bar to improve standards in primary schools and introduce a proper measure of progress from when children start school to age 11.
"Under our proposals to improve primary accountability, schools will be held to account either for ensuring all children make sufficient progress from reception to the end of primary school, or for ensuring at least 85% achieve the expected level in reading, writing and maths.
"We want to see all children leaving primary school with a good standard of reading, writing and maths so that they can thrive at secondary school."