Around 600,000 primary school children will not sit exams as teachers boycott the SATs.
Increasing numbers of schools are confirming they will not administer the exams with more expected to join them.
Two teaching unions are staging the boycott - the National Union of Teachers has said that more than 70 per cent of schools will be taking part in some areas.
Children aged10 and 11 were due to take the tests throughout this week, starting with reading.
The test results are used to determine league tables and Ofsted reports, both of which will be affected by the missed exams.
Teachers unions are insisting this will be industrial action with no downside as children will still be taught.
They claim the tests cause unnecessary stress to children and undermine education through the creation of league tables.
The action is expected to be the first big problem for the incoming Government.
