Fire fighters are holding unofficial industrial action against changes to an agreed pay deal.

Staff across the country are now set to defy their union by embarking on industrial action in protest at the proposed pay deal.

Fire fighters have announced they will only answer 999 calls as they commence a "work to rule".

Meanwhile the union's high command is considering on whether to call for further walkouts.

Employers have warned, however, that there will be no change to plans to implement the new pay deal in two stages.

They say the seven per cent award, which is due this month, was conditional upon modernisation moves which are not yet complete.

Under the deal fire fighters will receive a 3.5 per cent increase now while a similar pay rise next year will be backdated.

"Fire and rescue service employers today reiterated that the same conditions agreed by the FBU executive back in June, still apply to the release of stage payments to personnel in exchange for service improvements," an employers' statement said on Monday.

However, the FBU has said it cannot see what motives lie behind the move.

It is considering whether to take further action following a series of strikes in the past year.

"It is all backdated to November, so there is no savings in it for them," said a spokesman. "We don't understand why they have done it."

The Liberal Democrats warned the union against holding further action.

"Another fire fighter strike would be disastrous and has to be avoided," spokesman Ed Davey said.

"Liberal Democrats urge local authority employers and the FBU to try everything that will stop their disagreements from developing into a full-blown dispute.

"For the nation's safety, we hope that early indications of successful negotiations turn out to be sincere and that both sides will show genuine signs of good will."