By Hannah Goff
BBC News at the NUT Conference in Manchester
The government says strike action will disrupt pupils' education
Teachers have escalated their dispute with the government by voting to ballot on a campaign of industrial action over pay, workload and class sizes.
The National Union of Teachers (NUT) will ballot members on a series of possible walkouts in England and Wales.
The move came after delegates at the union's annual conference in Manchester condemned the prime minister's approach to limiting public sector workers' pay.
The NUT is already balloting on a one-day walkout over pay on 24 April.
The ballot closes at the end of the month and if successful will lead to the first national strike for 22 years.
Delegates have now endorsed a motion calling for a rolling programme of action if next month's strike is agreed.
It also calls for the NUT to draw up a strategy to protect teachers' salaries, reduce their workload and limit class sizes.
The vote gives the union the power to ballot its members on either a series of strikes or lower level industrial action, but the detail of what happens depends on how the second ballot is phrased and whether it is successful.
Ministers have announced a 2.45% rise for teachers in England and Wales this year, with rises of 2.3% in 2009 and 2010.
But the NUT claims this is a pay cut in real terms because it is below the true rate of inflation.
It wants the government to use the Retail Price Index for its calculations on public sector pay, rather than the Consumer Price Index which is lower.
Ian Murch, from the NUT's ruling executive, warned the PM: "If I were you Mr Brown I would be doing my sums again.
"You wouldn't like us when we are angry - and we are getting a bit angry now."
And he told Schools Secretary Ed Balls to prepare for a fight, adding: "If I were you Mr Balls, I would put my tin hat on right now."
NUT president Bill Greenshields said: "We don't do the job for money, but we can't do it without. If a society values its children, it will value its teachers."
A spokesman for the Department for Children, Schools and Families said: “Strike action can only achieve one thing - disruption to children's learning."
The 2.45% pay offer recommended by an independent review body was "fair" and above both the current CPI inflation rate and the government's inflation target, he added.
"Everybody understands, including teachers, that we need to have a firm control of public sector pay."
Meanwhile, the government had invested in cutting class sizes and the ratio of pupils to teachers been "substantially" reduced, ensuring children had more individual attention.
And support staff numbers had doubled over the last decade, freeing teachers from a range of administrative duties.
But Lewisham delegate Martin Powell-Davies warned: "Our members are ground down, our members are being forced out by workloads and low pay."
Radnor delegate Mary Compton said: "We are at last waking the sleeping giant which is our union's ability to take strike action and defend state education."
Another delegate from East London, Paul McGarr, said: "The government has the money to give us what we need - two words prove that Northern Rock."
He added that half a million teachers on strike and marches and rallies in every area could make the government change its mind.