London Fire Brigade's control room, which handles its 999 calls, will be privatised, London Fire and Emergency Planning Authority has said.
Members of the authority agreed to outsource the service and awarded a 10-year contract to Capita, which would save £5m over 10 years.
The authority said LFB would be able to focus on "putting out fires, rescuing Londoners and saving lives".
But unions said the move would put the public in danger.
LFB's control room handles about 2.5 million calls a year and it is thought to be the first in the country to be privatised.
Existing control room staff will become employees of Capita.
No date has yet been agreed for the change because the contract has to be finalised and signed, although it is expected to be ready by the end of 2012.
Councillor Brian Coleman, chairman of the London Fire and Emergency Planning Authority, said: "This is a win-win situation for Londoners.
"Outsourcing the brigade's 999 control centre will mean people in the capital benefit from a new, hi-tech system that will mobilise our firefighters to incidents even more quickly and this will be done at less cost.
"At a time when the public sector is having to look carefully at what it spends, this contract will represent excellent value for money for taxpayers across the capital."
But Vic Bagnelle, a control officer of 32 years' experience and spokesman for the GMB union, said that many experienced officers who did not want to work in the private sector would leave.
He warned that inexperienced "call centre staff" might not be as adept at the job.
Mr Bagnelle said: "They are playing with people's lives, livelihoods and properties.
"The service will suffer."