Fire crews will be unable to reach an extra 40 London wards within the six-minute target if plans to close 12 fire stations go ahead, it has been claimed.
London Fire Brigade (LFB) said the 40 wards would join 267 wards currently performing outside target.
Its figures indicate first response times would increase by over a minute in 34 wards.
But the brigade said over half of all wards in London would fall within the six-minute target.
The closure of more than 10% of fire stations and the loss of more than 500 jobs were proposed by London Fire Commissioner Ron Dobson in a bid to save nearly £29m over two years.
The plan is open to public consultation until 17 June.
LFB’s target is to get a first fire engine to an emergency within an average of six minutes and a second within eight minutes.
The document indicates the worst affected ward would be Clapham Town where the first response time would increase from nearly four minutes to nearly double that figure.
Green Party Assembly Member Darren Johnson, a member of the London Fire and Emergency Planning Authority (LFEPA) which governs the fire service, was among those who requested the figures.
He said it "proved" some local neighbourhoods will be "badly hit" by station closures.
He added: "These unnecessary closures are a result of the Mayor’s trifling 7p a week council tax cut.
"A delay in response times can literally mean the difference between life and death."
The Mayor of London’s office was approached to respond to this but did not.
Labour’s fire service spokesman on the London Assembly, Navin Shah, said the increases were "truly scandalous".
LFB’s data model uses fives years of incident locations and calibrates them against demand frequency and response performance in one year, 2011/12.
A spokeswoman for LFB said the proposals sought to keep London-wide response targets.
She said: "As is the case now, over half of all London wards would, on average, continue to get a first response within the six minute target if the proposals are agreed.
"London would continue to receive a very good service, compared to other emergency services and other parts of the country."