Up to 40 oxfordshire schools are expected to close or cancel lessons tomorrow as teachers go on strike over pay.

The one-day strike by the National Union of Teachers (NUT) is the first for 21 years and will involve about 600 of the county's teachers. There are 2,750 NUT members in Oxfordshire.

Last night, 25 schools were thought to be closing and another 12 will only have lessons for some pupils, but the union expects that figure to rise today to about 40.

Teachers are also planning a march through Oxford city centre from the Town Hall at 1.30pm tomorrow together with other public sector workers also protesting about pay.

President of NUT Oxfordshire Chris Blakey said: "We are not prepared to put up with below-inflation pay rises any longer.

"This is only the start of the fight over pay and demonstrates the frustration and anger felt by many."

Brenda Williams, secretary of NUT Oxfordshire, added: "We don't want children to lose a day's education but, in the grand scheme of things, there has to be a point at which we identify what is happening to the teaching profession."

Gawain Little, one of 10 teachers going on strike at St Ebbe's Primary School in Oxford, said: "We are striking for the future of the education system.

"The more we see teachers' pay eroded, the less we will see people going into the profession. The long-term effects are immensely damaging."

Parents greeted the strike with mixed feelings, some supporting teachers while others have voiced their anger.

Mum Helen Cauvy, who has a child at St Ebbe's, said it was "a pain", while John Howkins, whose daughter goes to Larkmead School in Abingdon, said: "I think they should do these strikes on inset days. They are simply denying all these children a day's education."

Oxfordshire County Council said it had liaised with every headteacher in the county about the industrial action, asking them to contact parents and the council as soon as a decision to close a school had been made.

Some schools sent out letters before the Easter holidays, while others only announced closures yesterday.

Children's Secretary Ed Balls said: "I'm on the side of parents who will be disappointed if their children's education is disrupted tomorrow, because we have decided to accept an independent pay review.

"The majority of teachers do not want to strike."

Members of the NUT last month voted 3-1 to strike over the Government's offer of a 2.45 per cent pay rise. The union leadership wanted at least 3.8 per cent to match inflation.

Mr Blakey said: "This would be the first year of a three-year pay settlement, which sets below-inflation pay rises until 2011 and forms part of the Government's attempts to hold down public sector pay."

He said this news had come at a time when teachers' workloads have risen out of all proportion, saying: "Stress-related illness among teachers has never been so high."

Figures show teachers work, on average, more than 50 hours a week, which Mr Blakey said means teachers are now being paid less for doing more.