Teaching unions predict more than half of England's 17,000 primaries will refuse to test 10 and 11-year-olds this week

Thousands of primary schools will boycott national tests for 10 and 11-year-olds tomorrow, treating their pupils to class trips and lessons in creative writing instead.

Teaching unions have predicted that half of England's 17,000 primaries will lock up their test papers in protest, affecting tens of thousands of pupils.

Some 600,000 pupils are due to sit the tests, known as Sats, in maths and English every day this week. Unions argue that the tests disrupt children's learning and are "misused" to compile league tables, which they say humiliate and demean children and their schools.

Teachers said that in some parts of England, such as Calderdale, Hartlepool, Barnsley and the London boroughs of Tower Hamlets and Kensington and Chelsea, more than half of all primaries have refused to administer the tests.

A survey conducted by the Press Association shows that in 37 local authorities alone, an estimated 1,010 schools have already said they will be boycotting the tests. More are understood to be still considering what action to take.

In Kirklees, 83 out of 152 schools will take part in the boycott, while in Dudley 50 out of 79 will. Manchester city council said half of its primary schools – about 60 – will be taking action.

The unions said a letter from Ed Balls, the schools secretary, warning school governors that it was teachers' statutory duty and professional responsibility to carry out the tests had backfired and spurred more teachers to join the boycott.

Both Labour and the Conservatives have insisted Sats should not be scrapped, although Labour has said the system is "not set in stone", while both the Tories and Lib Dems have promised reform.

Headteachers from across the country told the Guardian they would use the boycott to take pupils on trips and have classes in subjects such as creative writing.

Teachers in London have organised a giant anti-Sats picnic near the London Eye. Its organiser, Sara Tomlinson, predicts at least 20 schools will bring their classes. The children's author Alan Gibbons will tell stories and pupils will bring their favourite books.

Pupils at Bromstone primary in Broadstairs, Kent, will prepare for a local schools' writing competition while 10 and 11-year-olds at Lindale primary in Cumbria will spend their week going on school trips and being taught orienteering. Children at Westfield junior school in Hinckley, Leicestershire, will visit Beaumanor Hall, a stately home used for military intelligence gathering in the second world war. Other schools said they would continue lessons as normal, but without any test preparation.

Christine Blower, general secretary of the National Union of Teachers, said: "We know that schools will be using the boycott as an opportunity to do things they wouldn't normally have time to do, such as trips to museums and parks."

Nigel Utton, headteacher of Bromstone primary, said Sats were "unbelievably unreliable". "They are inaccurately marked, the quality of the questions is very poor indeed, they skew the curriculum horribly and they give unnecessary stress to the children. We already assess pupils ourselves."

Michael Rosen, the former children's laureate, told parenting site Mumsnet that Sats reduced children to machines and "units of productivity".

In a question-and-answer session on the site, Rosen wrote: "I think we are obsessed by giving kids scores, measuring them and producing research that is based on statistics. This biometric approach to human behaviour is to my mind corrupting. It tries to reduce the variability in human behaviour. The difference between humans and machines is that with machines, you can keep all the variables in your test constant ... you can't do that with human beings."

Children are tested and graded more than ever before, Rosen said.