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Lives are being put at risk because ambulances are taking up to three minutes longer to reach seriously ill patients.
NHS figures, highlighted by Labour, 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.
In the East of England, there has been an increase of two minutes and 48 seconds, though it still meets the response requirement of under 8 minutes.
The chief executive of the East of England Ambulance Service said the service was dealing with delays by recruiting extra paramedics and putting additional ambulances on the road.
Dr Anthony Marsh said: "We are putting more staff on the front line and reducing the number of cars to increase the number of ambulances.
We are recruiting 400 student paramedics by April 2015, the first group of which are already working from ambulance stations across the region, as well as up-skilling our existing emergency medical technicians and emergency care assistants.
Additional ambulances are also on the road, and we launched a replacement programme to ensure no ambulance in our fleet is older than five years by next spring."