Boris Johnson has overruled the governing body of London Fire Brigade and has given the go-ahead for plans to close fire stations and cut jobs.
Ten fire stations and 552 firefighters' posts are due to go as part of plans to save £28.8m.
The number of fire engines will be cut by 14. Minimum crewing levels on rescue units will go from five to four.
The emergency planning authority had narrowly voted to reject the proposals, however the mayor had the final say.
Mr Johnson has said he wants the plans implemented by 16 September.
It means London will be left with 155 fire engines at 102 stations. It employs about 5,500 firefighters.
Based on current spending and funding forecasts, London Fire Brigade expects a budget shortfall of £35m for 2014-15.
In a letter to London Fire & Emergency Planning Authority (LFEPA) chairman James Cleverly, Mr Johnson said action had to be taken so savings could be made and that further delays would increase the likelihood of compulsory redundancies.
London Fire Brigade Union (FBU) said the mayor's decision was an "affront to democracy".
Ian Leahair, FBU executive member for London, said: "The cuts are dangerous and wrong, and this is devastating news for Londoners, with lives across the capital being put at risk by the mayor's reckless cuts.
"[Boris] Johnson has simply ignored the evidence, and his cuts will mean slower response times for four million Londoners."
The plan will see 10 stations closed, 14 engines removed, 552 frontline jobs slashed and the removal of two rescue units.
Ninety four per cent of Londoners who took part in the public consultation opposed frontline cuts in the London Fire Brigade, and hundreds of Londoners also voiced their opposition at public meetings
The London Fire and Emergency Planning authority, the London Assembly and several local authorities also oppose the cuts, along with other political parties including Labour, the Greens and Liberal Democrats.
The proposals had been criticised by Labour, the Greens and Liberal Democrats.
London's Fire Commissioner Ron Dobson said: "We have to acknowledge that the number of fires we attend has gone down by half in the last 10 years, and our latest figures show that fires continued to fall at the same rate last year."
He added that "fire stations and fire engines do not stop fires happening - proactive prevention work does".
The LFB said it was now considering the mayor's direction.
Darren Johnson, London Assembly Green Party member, said: "The mayor has completely ignored the concerns of the majority of the London Assembly, the fire authority, and the 94% of residents who responded to his consultation by opposing any station closures."