Thousands of firefighters in the country's biggest brigade are to be balloted for industrial action in a row over new contracts, it has been announced.
The Fire Brigades Union (FBU) said its members in London will be asked if they want to launch a campaign of action short of a strike in protest at changes to shift patterns.
The London Fire Service announced earlier this month that it was starting consultations on terminating existing employment contracts and re-engaging firefighters on new start and finishing times to shifts.
FBU officials said ballot papers will be sent out next week after the brigade failed to meet a deadline of 5pm on Friday to withdraw the planned changes. Ballot papers will go out on August 27 and must be returned by September 17, with any action likely to start from September 24.
General secretary Matt Wrack said the "bombshell" from Commissioner Ron Dobson had "torpedoed" negotiations on changes to shift patterns, which the authority maintained had been continuing for five years.
Mr Wrack said: "His threat has not been withdrawn and we are left with no option but to ballot London's firefighters on industrial action.
"Firefighters hate taking industrial action and will only do so as a last resort. Their ethos is one of public service, but there is a limit to how much bullying they will tolerate, and the London Fire Service has overstepped that limit."
The FBU's senior official in London, Ian Leahair, said: "No-one in the FBU wants this conflict. I do not believe Ron Dobson really wants it, either - he cannot really want to do this sort of damage to the London Fire Service.
"I think his hand was forced by his political master, Brian Coleman, the combative Conservative chair of the London Fire Authority, who has been itching for a scrap with London's firefighters for a long time."
Mr Dobson said: "This is totally unnecessary. Both sides have agreed all along that discussions have been businesslike and, although no agreement has been reached yet, a settlement should and can be produced which will satisfy all parties within the next three months. These changes are about doing all that we possibly can with existing resources to make Londoners safe."