London's firefighters have been told they face being sacked if they do not agree to work new shift patterns.
London Fire Brigade has begun formal consultation arrangements to terminate all contracts and offer re-employment with new start and finish times.
The move, which would result in longer days and shorter night shifts, comes after five years of failed talks.
The Fire Brigades' Union (FBU) said the proposals were unacceptable and it would ballot members for strike action.
London Fire Commissioner Ron Dobson wrote to the government and the FBU on Wednesday informing them of his decision.
He said: "I want to use the consultation period to focus the ongoing meetings we are having with the FBU and reach an agreement.
"The discussions so far have been open and businesslike and I am confident that these negotiations can, and should, produce a settlement that will satisfy all parties and bring an end to this longstanding issue."
He said if an agreement with the FBU was reached during the 90-day consultation, the process would cease and "termination of contracts won't be necessary".
But the FBU said they thought the new shift system would be "extremely damaging".
An FBU spokesman said: "It will impact peoples' lives to the degree they may be forced to leave their job. It would also be a gateway to cutting fire protection across London."
He said they would look to ballot members on industrial action later this month.