London Air Ambulance (LAA) has launched an urgent fundraising appeal as it looks to raise £15m to replace two helicopters.
The charity said it needed to raise the funds by 2024 to replace the aircraft.
LAA, which gets 89% of its funding from public donations, is launching the campaign, Up Against Time, on Tuesday.
The group's medical director, Dr Tom Hurst, said without support some patients would not be reached in time.
Last year, LAA assisted 1,714 patients at the scene - an average of about five a day.
The charity is aiming to raise awareness of the campaign by publishing new polling data, which showed only 38% of Londoners know the service is primarily funded by public donations.
Thirty-six percent thought it was funded by the NHS or central government, while 33% of respondents did not know that LAA medics carry out procedures such as open-chest surgery, blood transfusions and reinflating collapsed lungs at the scene.
The poll also found that 51% of those surveyed thought the air ambulance only took patients to hospital, but the organisation said a key role was getting medics to the scene of patients who had suffered life-threatening or life-changing injuries.
Dr Hurst said: "When patients are so seriously injured there is no time to reach hospital, our medics give life-saving treatment at the scene - like open-chest surgery and blood transfusions.
"But we're a charity, and without public support we wouldn't be able to reach our patients in time to help them.
"Right now, we have an urgent need to replace our helicopter fleet - the current helicopters are becoming increasingly difficult to maintain.
"It's only with the backing of the public that we can continue to serve all those who live and work in London when they need us most."
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