By Angela Harrison
Education reporter, BBC News, at the NAHT conference
The Sats results are used to make league tables
Head teachers are to vote on whether to ballot members over a possible boycott of a centrepiece of England's education system, the Sats tests.
The National Association of Head Teachers, meeting in Brighton, says the tests put too much pressure on 11-year-olds and narrow the curriculum.
One of the biggest classroom teachers' unions, the NUT, has already said it will ballot its members on a boycott.
Ministers insist Sats are important in showing how local schools are doing.
The Department for Children, Schools and Families says a boycott of the tests taken in English, maths and science would be "irresponsible and illegal".
Heads and deputies in the NAHT will debate and vote on the issue later, immediately after hearing from Schools Secretary Ed Balls in person at their annual conference.
The tests are used to make the league tables, which are generally disliked by heads and teachers, who think they are unreliable and do not give a true picture of a school's achievements.
Tests are also taken by seven-year-olds in England, but only to bolster assessments made by their teachers. These are not used for league tables but would also be affected by any boycott.
Sats and league tables only exist in England. In other parts of the UK, children sit various tests but these are marked in schools and are not used to make tables.
The tests are marked externally and the unions say this is at great cost.
They have perhaps been emboldened in their opposition to the tests by last year's fiasco over marking and the delays to the results of hundreds of thousands of children's papers.
The government's decision in the autumn to scrap compulsory Sats tests for 14-year-olds may also have had an impact.
Ministers are reminding heads that they have a statutory duty to administer the tests - something not disputed by the NAHT - and say any boycott would be both irresponsible and illegal.
Mr Hill says he is likely to vote for a boycott
A previous statement on the issue said: "These tests are important as they allow parents to see how their children and local schools are doing. Any attempt to boycott them could undermine this and risk removing a basic right from parents".
The government has set up an "expert group" to advise on future testing and assessment arrangements which is due to report in "late spring".
Head teacher Chris Hill, from Hounslow Town School in west London, says he will listen to the arguments but will probably vote for a ballot.
He says the tests only confirm what the schools already know.
"At the moment, we are preparing for the national tests. We are putting the children under a lot of pressure because we want them to do their best. However, there are a lot of other ways we get the information. We track their progress throughout the year so we really know what the children are capable of doing.
"Also the tests distort what we do in class because we concentrate, and rightly so, on literacy and numeracy but there is a lot of the other curriculum which gets squeezed because of the desire to get the best results we can achieve."
He agrees that children need a challenge but says the pressure is excessive.