Changes to the 999 call handling system before the Olympics and the Queen's Diamond Jubilee could save at least 150 lives each year, the Department of Health has said.
The overhaul will ensure the most critically-ill patients are prioritised and should save ambulances from making more than half a million wasted journeys a year.
It is hoped the new system will keep many more vehicles available for the most serious emergencies, increasing the chances of survival for anyone who suffers a cardiac arrest or stops breathing.
The changes are to be brought in before events such as the Diamond Jubilee and the Olympics, which are expected to heap pressure on the emergency services.
From next month, staff answering 999 calls will categorise patients using a two-tier system.
In critical cases, the Government will expect ambulance trusts to reach the casualty within eight minutes. Where a patient's condition is not deemed life-threatening, the call handler will be given up to 60 seconds longer to collate more information before dispatching help.
These "Red 2" calls are designed to ensure staff send the most appropriate response vehicle to patients, meaning fewer ambulance journeys are wasted or cancelled and more resources remain available for patients in greatest need.
Peter Bradley, chief executive of London Ambulance Service and National Ambulance Director, said: "At the moment, too many ambulances or rapid response vehicles are sent out when they are not needed.
"This means that paramedics are sent out on a double dispatch, only to be cancelled when they are too far away to reach another urgent call."