A proposed boycott of primary school Sats is under threat because of widespread concern among headteachers that it is illegal, union officials have admitted.
There is "a lot of nervousness" among heads and work needs to be done to win their support, said Steve Iredale, who will propose the boycott at the National Association of Head Teachers' conference in Brighton tomorrow.
Mr Iredale, who chairs the union's curriculum and assessment committee, said it would be a "tough call" to win over the majority of members if a national ballot were held. "Unions are often good at talking, but when it comes to the crunch it is very difficult, especially for headteachers," Mr Iredale told The TES. "Heads are vulnerable because they have a statutory duty to deliver the tests. We would not consider any action if we believed it would put members at risk.
"There is a lot of nervousness about doing something the heads believe is right, but is potentially illegal. It is trying to persuade those people. If it goes to a national ballot, we will need to escalate the whole thing to get members on board. It would be a tough call."
The NAHT vote follows unanimous support given to a potential boycott at last month's NUT conference. The two unions are working together to campaign for the abolition of national tests in primary schools and are proposing a boycott of next year's Sats unless the Government implements radical reforms of the testing regime. About 80 per cent of primary school heads are members of the NAHT.
Mr Iredale, head of Athersley South Primary in Barnsley, South Yorkshire, said he was confident the motion would win support. But to go ahead, the proposal needs backing in a national members ballot.
It is understood that union officials are considering different forms a boycott could take. Schools could administer the tests but not send them to be marked. This would mean heads had fulfilled some of their statutory duties, but would still disrupt the publication of school league tables.
Mr Iredale said he had a "passionate hatred" of the narrowing effect that Sats have on the primary curriculum. "I can see the damage being done to children and think enough is enough," he said. "The profession has talked for long enough and it has got us nowhere."
The NAHT and NUT want league tables to be scrapped and tests to be replaced by teacher assessment. They are calling for the introduction of a national sampling scheme to monitor progress in English, maths and science. Both unions have said a boycott would be a last resort.
Ed Balls has said that he will wait for an "expert group" report on the future of assessment before making final decisions on changes to Sats, but has said assessment will remain.
The Schools Secretary, who is due to address the conference on Sunday, could face hostility. He has criticised the proposed boycott as "irresponsible" and "retrograde".
Mick Brookes, general secretary of the NAHT, writing in today's TES said that the accusation was "an easy slur and a cheap shot".
"What is irresponsible about wanting to see an end to the annual humiliation of children, their teachers and their communities in the publication of league tables?" he writes. He has described government claims that the unions want to withhold information from parents as "deliberately misleading, insulting and outrageous".
Urgent action is needed to reform the top level of local authority staffing to ensure that there is a position dedicated to overseeing schools as well as childcare, the NAHT conference will be told.
Attention to education is being "diluted" when local authorities place too much emphasis on safeguarding the child, one delegate will argue.
Gillian Mitchell, who teaches at New Silksworth Infant School in Sunderland, says the union wants to retain the position of director of children's services, but is calling for a change in the law to ensure that all councils have a director of education as well. Without one, schools risk losing specialist support for human resources, finance and school improvement, she believes.