Lessons might now be affected directly

From 15 March, members of the NASUWT will refuse to cover absent colleagues' lessons, on top of action that has been hitting out-of-hours meetings.

In a ballot, 94% backed the action and 79% voted to strike - though there are no plans for that at this stage.

The dispute involves pay comparisons with teachers in England and Wales.

NASUWT has sought, to date, to avoid taking any action which could impact on pupils but we have been left with no choice Union leader Eamonn O'Kane

Teachers in Northern Ireland complain they lost out when those in England and Wales had access to the latest tier on the new, higher, performance-realted pay scale.

An independent review finally recommended pay rises backdated only to September 2003, not 2002.

The unions say this meant their members lost about �1,000 apiece.

Talks

But also it meant they were no longer in step with England and Wales - in effect setting up a regional pay difference to which they are opposed on principle.

The employers dispute the notion that it is about parity.

They argue the unions rejected the same performance-related scheme as in England and Wales and in effect were seeking "money first and reform later".

The other main teachers' unions in Northern Ireland - the Association of Teachers and Lecturers, the Irish National Teachers' Organisation and the Ulster Teachers Union - have also been working to rule.

The employers have offered conciliation talks at the Labour Relations Agency (LRA) but the unions say they are not being "serious".

A spokesperson for the employers' side said their understanding was that the matter was still in the hands of the LRA.

"We would like to see a resolution through that, and any intensification of the dispute is unhelpful to all concerned."

The NASUWT has about 9,500 members in the province's schools, some 45% of whom took part in the ballot.

Its general secretary, Eamonn O'Kane, said the result - with almost 94% of those who voted backing action short of a strike - showed "the very deep sense of anger and frustration felt by members over the failure to maintain pay parity".

"NASUWT has sought, to date, to avoid taking any action which could impact on pupils but we have been left with no choice.

"We remain willing to enter into serious talks with the employing authorities to seek to resolve this issue."