School governors have attacked the boycott of England's Sats tests by primary school heads and deputies, saying it is "disappointing".
Two unions that together represent most of England's primary head teachers announced on Wednesday afternoon that the action would go ahead.
Half a million children have been preparing for the tests, due to be taken days after the general election.
The National Governors' Association says a boycott will damage schools.
It has also questioned the legality of the dispute.
All the political parties are against the boycott and ministers say heads who refuse to stage the tests will be in breach of their statutory duties.
There was a majority vote in favour of the action from those who voted in ballots carried out by the National Association of Head Teachers and among members of the National Union of Teachers who are heads or deputies.
The results were released on Friday but the unions left it until Wednesday to announce whether or not the action would go ahead.
The tests - in English and maths - are used to make up the school league tables.
This decision to go ahead with a boycott will throw schools and councils into confusion.
If the action is taken and thousands of children do not sit the tests, this year's league tables could be meaningless.
Not all heads or deputies will be members of the unions behind this action and it is possible a deputy head or other senior teacher could run the tests.
Governors are not meant to get involved in "operational matters" so could not step in to stage the tests themselves.
The National Governors' Association says the boycott will damage relationships in schools and be "extremely problematic".
The organisation's chair, Clare Collins, said: "We are very disappointed. We had hoped that at the last point they would decide not to take the action.
"We don't know how many people are going to obey the call to action, we don't know how many will decide to administer the tests but not report the results, we don't know how many will just go ahead."
It was going to be hard, she said, for schools to tell children who had prepared for the tests that they would not now take place.
The group is calling on the Department for Children, Schools and Families (DCSF) to give clear guidance to governors.
"We believe that this boycott has the potential to damage relationships between heads and governors and between heads and their staff," she said.
"It is not clear that it is a legitimate trade dispute. The only way it can be determined is through the courts - and that is not something [ministers] would want to be in the middle of during an election."
The NAHT insists this is a legitimate trade dispute, arguing that head teachers' terms and conditions are adversely affected by the tests, particularly because of the pressure they feel over league tables.
Its leaders have called on ministers to meet them at the arbitration service Acas for talks.
They say the tests are bad for children - saying they narrow the curriculum - and bad for schools because of the influence of the league tables.
A DCSF spokeswoman has said the door there is also open for further discussions.
Netmums co-founder Siobhan Freegard has said it is too late to abandon the tests now, because the children have put in so much work for them.