Lives are being put at risk because ambulances are taking up to three minutes longer on average to reach the most seriously ill patients than three years ago, according to Labour.
Chaos at accident and emergency departments means ambulances are trapped in queues outside, which has a knock-on effect on the time it takes to deal with 999 calls , the party said.
It pointed to records published by the Health and Social Care Information Centre that show category A ambulance callouts, which relate to life-threatening conditions such as heart attacks and strokes, are taking an average of 67 seconds longer than in 2011.
That rises to two minutes and 21 seconds in the East Midlands, taking its response time just over the target eight minutes. In the East of England, there has been an increase of two minutes and 48 seconds, though it still meets the response requirement.
Dale Webb, director of research and information from the Stroke Association, said: " When stroke strikes, the blood supply to part of your brain is cut off by a clot or damaged by a bleed which causes brain cells in the affected area to die, so time lost is brain lost. Lengthening ambulance times are concerning because stroke patients need to get to specialist treatment as soon as possible.
"T he quicker someone arrives at a specialist stroke unit the quicker they should receive the right treatment and the more likely they are to make a better recovery."
Shadow health secretary Andy Burnham claimed the figures were proof that Prime Minister David Cameron "can't be trusted with the NHS".
He said: "These figures raise real concerns that lives are being put at risk by the chaos in the NHS.
"Hospital A&E departments have missed the Government's waiting time target for the last 51 weeks running. This crisis in A&E has trapped ambulances in queues outside hospitals - leaving the next caller facing longer, agonising waits. More and more calls are being attended by police cars and even fire engines on David Cameron's watch.
"For people who've suffered cardiac arrest or a stroke, every second counts and that is why this slump in standards cannot continue.
"Urgent action is needed from the Government to turn things around. Slower 999 response is yet another sign of an NHS heading in the wrong direction under this Government. Their failure to get a grip on the A&E crisis is now having a knock-on effect on the safety of our ambulance services. It is more proof that David Cameron simply can't be trusted with the NHS."
A Department of Health spokesman said: "We know some ambulance trusts are under pressure, carrying out over 850 more journeys every day since 2010, but the NHS continues to respond to the majority of the most life threatening cases in less than eight minutes.
"We're giving the NHS extra support to keep services sustainable year-round and where performance has been poor, action has been taken to address longstanding problems with new chief executives brought in at East Midlands and East of England ambulance services with clear recovery plans in place."