Crews have eight minutes to respond to life threatening emergencies
The government has been urged to review its targets for ambulances responding to 999 calls, following claims that patient care is being affected.
The NHS Confederation, which acts for ambulance trusts, said that targets can produce "unintended consequences" and "may not be benefitting patients."
Ambulances have eight minutes to respond to 999 calls for emergencies such as heart attacks or strokes.
However, Health minister Mike O'Brien said targets "drive improvement."
Government targets state that ambulance trusts must respond to 75% of category A 999 calls (life-threatening emergencies) within eight minutes.
There are other target times for categories of less serious calls.
But Nigel Edwards of the NHS Confederation, which represents ambulance trusts, told BBC Radio 4's The Report programme that the NHS needed to move away from "very narrow process type targets".
The only proven clinical value of an eight-minute response is for patients with cardiac arrest, where a really fast response really can make the difference between whether they survive or whether they die
Janettte Turner, Sheffield University
He said: "The clinical basis for the eight-minute target is pretty secure, the difficulty is that the target starts from when you answer the phone, when there's quite a lot of information to be captured at the start, but the basic principle that you should try to get ambulances to people with life threatening conditions is I think the right one."
However Mr Edwards added: "Any narrow target which focuses on one measure does have the potential for producing unintended consequences and maybe not benefitting patients.
"A solution might be to move to a measure in which we measure the outcome of what's done rather than just the process, so what was the outcome for the patient? Did they receive the emergency care that they should have done?
"And if we could move to a situation which was better at measuring that, then we might avoid some of the unintended consequences of these very tight timescale based targets."
He was backed by academic Janette Turner, from the Sheffield University's Medical Care Research Unit.
She told the BBC: "The only proven clinical value of an eight-minute response is for patients with cardiac arrest, where a really fast response really can make the difference between whether they survive or whether they die, but for the other patients there's no proven relationship between how quickly the ambulance gets there and whether they survive.
"The problem that creates for ambulance services is if they get there in seven minutes and the patient dies, they have succeeded because they have met a target and if they get there in nine minutes and the patient lives, they have failed because they haven't reached the target."
She is now working with the Department of Health on developing additional performance measures.
Health Minister Mike O'Brien said: "Targets help to drive improvement and we have seen ambulance trusts making progress following the changes to performance requirements from 1 April 2008.
"Investment and improvements in ambulance services include increases in frontline staff and emergency medical dispatchers as well as new vehicles, equipment and technology so patients get a better quality, faster service.
"The 2008 Healthcare Commission patient survey showed that 97% patients were satisfied with the ambulance service - one of the highest ratings ever received by any NHS service."
The Report is on BBC Radio 4 on Thursday, 17 December at 2000 GMT. You can also listen via the BBC
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