Teachers are to be balloted on a strike over pay, their union said on Friday, a move that could result in the first national walkout by teachers in two decades.

The National Union of Teachers' decision to threaten strike action, set for late April, dramatically escalates their pay row with the government and creates another industrial headache for Prime Minister Gordon Brown in a week when more than 20,000 police officers took to the streets of London to protest pay.

The NUT confirmed that if taken, it would be the first national walkout in 20 years.

The government said if such a move were taken it would be an unnecessary disruption of children's learning.

The NUT's proposal, which will be put to teachers late next month, came after a three-year pay deal was announced by the government last week and rejected by the union.

The schools secretary, Ed Balls, said teachers salaries would rise by 2.45 percent from September 2008 and by 2.3 percent from September 2009 and 2010.

The 2.45 percent rise is higher than government's targeted Consumer Prices Index inflation rate of about 2 percent, although it is lower than the Retail Price Index - at present 4 percent - on which most private sector pay deals are based.

The threat of strike action is another blow to Brown, already reeling from a bitter pay row with police and after MPs accepted a below-inflation pay rise.

The NUT's general secretary, Steve Sinnott, confirmed that if teachers agreed to strike it would be the first walk off since 1987 which was a "significant development".

"It shows the deep concern there is within the public sector in the country over the issue of pay and deep concern from the teachers as well," he told Reuters.

He said teachers did not want to have any pay rises wiped out by inflation like the "bad old days".

Sinnott said he was confident that teachers would vote for industrial action and called on the government to rethink their offer. "I urge them (teachers) to vote yes," he added.

In contrasting views, the NASUWT union and the Association of Teachers and Lecturers have said the award meant teachers had done better than other public sector workers.

The head teachers' union, the Association of School and College Leaders, also said it had already approved the deal.

A department of schools spokeswoman said in a statement they were disappointed by the decision, which they said would disrupt children's learning.

"As teachers and NUT members see the details of the award we think they will agree it is fair and responsible in the current economic climate," she said.

"The 2.45 per cent award for teachers, recommended to us by the independent pay review body, is above the current CPI inflation rate and the Government's inflation target.

"Everybody understands, including teachers, that we need to have a firm control of public sector pay in order to keep inflation low, mortgage rates low and the economy stable."