Teachers are preparing to vote on a boycott of Sats tests which could spell chaos for next year's exams.
The National Union of Teachers (NUT) will on Saturday ask its members to agree a resolution calling for a ballot on industrial action which could mean teachers refuse to prepare or invigilate for the exams next year.
The Department for Children, Schools and Families said the proposed boycott would be "unlawful" and urged the NUT to scrap the proposal.
Speaking as the NUT's annual conference began in Cardiff, general secretary Christine Blower said she was "very confident" the resolution would be passed.
But Ms Blower added that she still had some hope that the Government would scrap the tests and action could be avoided.
She said: "I am very confident, and I expect that we will pass the motion. What we will do is keep our eye on what's going on, on everything Ed Balls might say, and everything the expert group might say."
The expert group, set up by the Government last year to look at the tests - which are taken by 11-year-olds in English, maths and science - is due to report back in a few weeks.
Schools Secretary Ed Balls this week said the current system was not set in stone and that he would be "surprised" if expert group said that it could not be improved.
Ms Blower said: "I thought he was saying quite helpful things. The assessment arrangements are not set in stone, that's something I was very pleased to hear. It was a clear indication there is every possibility that there will be some kind of change."
