Tens of thousands of primary school children are expected to miss their Sats today as a boycott of the tests kicks off across the country.
Rising numbers of schools in England have confirmed they will be joining a boycott, with headteachers at more than 1,000 schools already informing their local authorities they will not be administering the tests. In some areas, all schools are taking part, and in others more than half plan to take action.
Many more are expected to take part in the boycott, but have not told their councils.
The industrial action was confirmed by the National Union of Teachers (NUT) and the National Association of Head Teachers (NAHT) last month after ballots of their leadership members showed support. It is now down to individual heads, deputies and assistant heads to decide whether to take part.
A survey conducted by the Press Association shows that currently, in 37 local authorities alone, an estimated 1,010 schools have already said they will be boycotting the tests. More are understood to be still considering what action to take.
Ahead of the boycott, the NUT was yesterday holding an "anti-Sats picnic" in central London, which it said would be a celebration of reading.
The unions have raised concerns that Sats do not put enough emphasis on reading of whole books. Author Alan Gibbons was due to attend, with children urged to bring their favourite books.
Among those with high numbers of schools boycotting the tests are Barnsley in South Yorkshire, where the council said more than half of its 78 primaries are taking part.
In Manchester, 60 schools have confirmed they will be backing the action, and all 31 schools in Hartlepool. In Hertfordshire up to 32 schools could be boycotting.
Calderdale Council is estimating that around 75 per cent of its primary schools will be affected, and in Barking and Dagenham 16 schools are expected to boycott.
Some 21 local authorities said they do not yet have figures, with several saying schools have not been in touch with them. Five said they were aware of no primaries taking part in the boycott in their area.
The local authorities which said they expect the boycott to go ahead in some of its primaries are: Reading, East Sussex, Windsor & Maidenhead, Poole, Essex, Birming-ham, Kent, Hampshire, Calderdale, Rotherham, Kirklees, Hounslow, Gloucestershire, West Berkshire, Lincolnshire, Darlington, Doncaster, Norfolk, Enfield, Middlesbrough, South Tyneside, Stoke on Trent, Southwark, Bromley, Portsmouth, Barnsley, Swindon, Hertfordshire, Peterborough, Lambeth, Manchester, Hartlepool, Westminster, Barking and Dagenham, Bexley, Kensington and Chelsea, Harrow.
It is understood that primaries do not have to inform their council if they are taking part in the boycott, only the school's governing body must be informed.