Thousands of teaching assistants and support staff walked out
At least 4,000 schools in England and Wales have been closed by a two-day strike, its organisers have claimed.
Teaching assistants and support staff in England, Wales and Northern Ireland have been taking part in the walk-out by local government workers over pay.
Union Unison said the 4,000 figure only covered schools in England's North West and North East, Yorkshire and Wales, and that it was awaiting more details.
The Local Government Employers said 1,100 schools were closed on Wednesday.
However, Unison said it knew of 800 schools close by the action in Wales alone.
Members of three unions have been taking part in the strike over a below inflation pay offer of 2.45%.
Unison general secretary Dave Prentis said: "With inflation at an 11-year high and set to rise further, local government workers staff and their families cannot take another year of pay cuts.
"The employers need only to dip into their £11 billion reserves to end this dispute - they do not have to resort to cutting services or raising council tax."
Unison says the strike has had such an impact because of the fact that teaching assistants and support staff, many of whom it represents, now outnumber teachers and head teachers in schools.
But Local Government Employers said its figures from local councils suggested support for the pay strike was falling.
Returns from local authorities employing a third of the 1.3 million workers affected by the dispute have shown that 6.45% of staff in England, Wales and Northern Ireland are striking. a spokesman said.
This equates to around 84,000 - down from 100,000 yesterday, he added.
The strike comes as the pay review body that looks at teachers' pay announced it was not going to ask the Secretary of State for permission to review the 2005-7 pay award for teachers.
As part of that deal, a review could be triggered if the rate of inflation climbed above 3.25%.
Rising fuel and food costs pushed inflation up to 3.8% in June, according to the Consumer Price Index.