Pupils across England are taking alternative exams instead of being entered for government-backed Sats tests for 10 and 11-year-olds.
Senior teachers said it would enable schools to judge pupils’ progress while ensuring results could not be checked against targets or used to publish league tables.
The disclosure came on the first day of a week-long boycott of Sats.
Up to 300,000 children were prevented from sitting an official reading exam on Monday, with tests in spelling, writing and mathematics being targeted later this week.
The action is being led by the National Union of Teachers and the National Association of Head Teachers.
Unions claim Sats tests narrow the curriculum as schools are forced to drill children in the three-Rs to hit targets and inflate results. It is also claimed that the tests threaten head teachers' jobs.
Mick Brookes, NAHT general secretary, said the protest could be repeated next year unless the new government abolished Sats altogether.
A survey of councils by The Daily Telegraph suggested between a third and half of schools were boycotting the tests this week.
Birmingham Council – England’s biggest urban authority – said 153 out of 275 primaries confirmed they were taking action but actual numbers could be even higher.
Other councils including Calderdale, Barking and Dagenham, Darlington, Hartlepool, Kirklees, Middlesbrough, Stoke on Trent and Torbay indicated that the majority of schools were also taking part.
In many areas, schools are allowing children to sit alternative exams.
In Preston, 33 primaries are setting their own tests, which will be marked in school before being externally checked.
David Fann, head of Sherwood Junior School and a member of the NAHT national executive, said: “We are not opposed to testing per se, just the way these results are used to humiliate and demoralise schools, pupils and the communities they serve."
Schools in Bristol are allowing pupils to take old test papers, which are then moderated by neighbouring schools.
A similar system has been set up by primaries in Scarborough.
David Evans, head of Barrowcliff Primary School, said: “We need to remove this pressure cooker atmosphere that Sats create for children as young as 10, because it's just not reasonable and it doesn't make sense to pillory schools for the standard of the children that they teach.”
Mr Brookes said: "We think Ofsted will think this is a perfectly valid thing to do and parents are likely to be quite happy with this."
He insisted that the industrial action could be repeated in 2011 and 2012 if the incoming government refused to axe Sats.
Christine Blower, NUT general secretary, said: “Thousands of schools across England have decided that enough is enough and have taken the decisive step of boycotting this year’s SATs.
“There are reports from many areas that a significant majority of primary school pupils will not be sitting [Sats] tests this week. This will make the annual ritual of naming and shaming schools through league tables impossible.”. . . .