Fewer 999 calls will be regarded as life-threatening under an overhaul of ambulance response targets.
Current targets across England are being scrapped in favour of a new system which officials say will save more lives.
At present, half of all 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 and a maximum of 15 minutes overall for 90% of patients. These calls are still classed as life-threatening.
This means millions of patients, such as those with suspected heart attack and some strokes, will now see the target for them relaxed.
They will move from the life-threatening category to an emergency category.
These patients will be expected to receive a response within a maximum of 40 minutes for 90% of patients and typically within 18 minutes.
NHS England bosses said the move will save lives and that, at present, many calls classed as life-threatening turn out not to be.
A quarter of patients currently classed as life-threatening are not transported to hospital.