Firefighters in Wales are refusing to operate special units designed for use in the event of a major terrorist attack.
Firefighters during the last strike
They blame the decision on problems with an agreement made to end the industrial dispute in 2002.
Union leaders said members had only agreed to go ahead with training on the incidence response units as part of the deal, which was now on hold.
No-one from the fire service was available to comment on the move.
The units, which contain mass decontamination equipment, were being taken out of service by the South Wales Fire Service and the Mid and West Wales Fire Service from 1800 BST on Wednesday following meetings across the three services in Wales.
North Wales Fire Service were expected to follow suit within 24 hours.
Members of the Fire Brigades Union made the decision following a unanimous vote on Tuesday at their annual conference to withdraw from implementing the agreement reached in 2003.
Union officials said they had met all of its obligations under the pay and conditions agreement made in June last year.
They are angry that a 3.5% pay rise due to be backdated to last November is being held up.
The FBU has warned of fresh UK-wide industrial action.
Dick Pearson, a union official in Wales, said the units would be "off the road" following the decision to refuse to crew them.
FBU spokesman Mark Watt from Cardiff Central fire station said union members had withdrawn from every aspect of the pay agreement.
"The emergency resolution proposed by the executive committee yesterday was passed and everybody went back to their brigades.
"We'll recall conference in four weeks and decide if we will ballot members over industrial action," he told BBC News Online.
"By the weekend, every brigade in the UK will be [withdrawing the units].
He said it was not just the IRUs themselves that would be affected.
Eight fire engines are designated to accompany every IRU which responds to an incident.
Alan Richardson, assistant chief fire officer for South Wales Fire Service, said he was concerned about the withdrawal of cover, but stressed that firefighters were responding to every other type of incident as normal.
The last fire strike began in November 2002 over a £30,000 pay claim.
After months of protracted discussions following the eight-day walk-out, an agreement was reached which stated an average salary would be £25,000 by this summer.