It is believed that up to 3,200 lives might be saved each year if ambulances could reach 90 per cent of the estimated 300,000 urgent cases within an eight-minute period.
Emergency calls are dealt with in rotation, and the current target-response times are 14 minutes in town and 19 minutes in rural areas, regardless of the nature of the emergency.
Under the scheme to be introduced in all areas from October 1997, emergency operators will be trained to ask the caller a series ofbrief questions in order to determine the seriousness of the situation.
Life-threatening cases will include people who are unconscious, suffering from severe breathing problems, trauma with penetrative injuries, serious allergic reaction, and problems with children less than two years of age.
John Horam, the junior health minister who made the announcement, rejected union claims that the initiative would lead to the rationing of ambulance services, and that without extra funding, it would have a detrimental effect on the 999 service as a whole.
A spokesman for Unison, which represents 80 per cent of ambulance workers, said: "What we don't want is an elite service rushing around in helicopters and on motorbikes.
"We don't want a service that just deals with a small number of calls. If there is no new money it is difficult to know where any improvement is going to come from, unless another part of the service is to suffer."
Mr Horam said the new service's cost would be met by "improvements in ambulance service performance" and "extra investment over time from health authorities using their increased resources".
Speaking on BBC Radio 4's, The World this Weekend, he said he would "guarantee" people would still get their existing service, but the response to people whose lives were threatened would be speeded up to within eight minutes, "and that is an improvement."
Other organisations gave the scheme a cautious welcome. Professor Brian Pentecost, medical director of the British Heart Foundation, said: "Prompt arrival of skilled paramedics may make the difference between life and death for heart-attack victims."
The National Association of Health Authorities and Trusts described the move as "good news," but echoed union fears about resources. Philip Hunt, director of NAHAT, said: "Without extra resources health authorities will be hard put to find the investment needed from existing funds."