Fewer 999 calls will be regarded as life-threatening under an overhaul of ambulance response targets.
Targets across England are being scrapped in favour of a new system which officials say will lead to faster treatment for those needing it, saving 250 lives per year.
The move means millions of patients with conditions such as stroke or heart attack - who were previously expected to have a paramedic on the scene within eight minutes - will face a wait of up to 40 minutes.
Experts from the University of Sheffield, who conducted a study into the move by NHS England, said there was no evidence it would lead to patient harm.
At present, half of 999 calls for an ambulance are considered life-threatening, and a paramedic is expected to be on the scene within eight minutes.
From this autumn, the most serious calls, such as when a person is not breathing or their heart has stopped, will be expected to have a response within seven minutes typically, with a maximum of 15 minutes for 90% of patients.
Around four million calls a year will come under more relaxed targets and be classed as "emergency".
These patients will be expected to receive a response within a maximum of 40 minutes for 90% of patients, and typically within 18 minutes. At present, the target is eight minutes.
NHS England bosses said many calls classed as life-threatening turn out not to be.
Around a quarter of calls classed as life-threatening do not result in the patient being transported to hospital, they said.
The 18-month Sheffield trial on four million calls, involving ambulance trusts across England, found that up to 750,000 more vehicles will be available to immediately respond to emergency calls each year under the changes.
Some 6.5% of the most serious calls received a faster response than at present, and more patients who needed to go to hospital got an ambulance first time.
The Sheffield researchers said there were no reports of any safety issues for patients and no patient came to any harm from the changes.
Under the move, 999 call handlers will be given up to four minutes to assess what is wrong with a patient, with new questions assessing whether somebody is conscious or breathing.
At present, an ambulance is automatically dispatched 60 seconds into a 999 call.
Professor Keith Willett, director for acute care for NHS England, said the evaluation programme had shown "that we can achieve a faster response to those who are the sickest and those with immediately life-threatening conditions".
It had also shown "that all patients can get the response they need rather than the nearest response and, importantly, that the unacceptable lengthy delays in the system currently can be reduced".
He added: "What's wrong with the service now? We are still asking the ambulance services to operate on standards that have not changed since the 1970s."
He said that of calls currently classed as category 2 life-threatening, less than 6% of patients "actually benefit" from an eight-minute response.
"We've got a massive overreaction - or response if you like - to what is a clinical issue," he said.
"The result of having such a blunt target is that the ambulance service has to behave in a way that is not ideal.
"They will frequently have to deploy several vehicles to try and get one to stop that eight-minute clock.
"The result is that one in four vehicles that goes out on blue light and sirens is stood down either because another vehicle has got there first or we find out that that was not required because of the severity of the patient."
Prof Willett said increasing numbers of rapid response cars and motorbikes being sent was "inefficient" because a patient may still have to wait for an ambulance to be sent to take them to hospital, leading to "hidden waits".
Professor Jonathan Benger, national clinical director for urgent care at NHS England, said the new categories were set down according to patient need.
Some patients with chest pain and stroke will wait longer, but an ambulance will be sent to get them to hospital for the treatment they need, he said.
"Historically, the problem we've had is that a stroke patient might get a car response... then have to wait for an ambulance to take them to hospital," he said.
The most serious calls - such as choking, drowning, asphyxiation, cases where the heart has stopped or somebody is not breathing, and complications in childbirth that could harm a child - would get a response typically within seven minutes, he said.
In the Sheffield study, at one pilot site, cardiac arrest patients received a response 30 seconds quicker than previously.