Firefighters are set to launch a campaign of industrial action on Friday in a row over new contracts, with the threat of strikes next month.
The Fire Brigades Union said the action will go ahead in London from midnight on Thursday night unless the capital's fire brigade withdraws plans to introduce the contracts, which include different shift patterns.
The action will include an overtime ban, a refusal to undertake higher duties and a withdrawal from various voluntary projects.
Thousands of FBU members are also being balloted on strike action, with the result due on October 14. The union would have to give seven days notice of any walkouts.
FBU general secretary Matt Wrack said: "Firefighters hate taking industrial action, and what we want to do is negotiate, not strike.
"We understand that the London Fire Brigade wants to change the shift patterns our members work, and we are willing to continue the negotiations over shift patterns in good faith. We would be also be willing to lift all industrial action as soon as they withdraw the sacking notices."