More A&E departments closing their doors to ambulances
Hospitals had to close their doors to ambulances almost twice as often this winter compared with the previous three years, figures show.
The number of ambulance diverts in place at A&E departments in England hit 478 for the three-month period December to February.
This compares to an average of 249 times over the same period in 2013/14, 2014/15 and 2015/16, a report from the Nuffield Trust showed.
The charity said the delays caused by the closures was stopping ambulance crews responding quickly to other urgent calls.
None of the three main ambulance targets for responding to calls have been hit since May 2015.
Of the 233,472 most serious category A calls received in January this year, at least 95% should have had a response within 19 minutes.
However, the data showed that 29,000 calls did not receive a response within this timeframe. Just 88% of responses met the target.
Five NHS trusts accounted for more than half of all the 493 ambulance diverts reported this winter, with four in the North of England.
These were Pennine Acute Hospitals, Northumbria Healthcare, County Durham and Darlington and South Tyneside.
Worcestershire Acute Hospitals in the Midlands also accounted for a high number.
Professor John Appleby, Nuffield Trust chief economist and lead author of the report, said: "There's rightly been a lot of focus recently on the delays ambulance crews face in being able to hand over their most seriously ill patients at A&E.
"But what our research today has uncovered is the huge increase in the number of times this winter ambulance trusts have been told that they must take patients to another hospital altogether, because an A&E unit simply doesn't have the capacity to accept any more patients."
He said the pressure on services was affecting staff.
He said: "Even allowing for the inherently stressful nature of the job, our analysis of the results of this year's NHS Staff Survey shows they feel significantly more pessimistic than other staff in the health service about their ability to do their job properly and provide a good service.
"Managers in the NHS and politicians need to make improving the ambulance service's poor morale and its ability to meet targets an urgent priority".
The report said the number of category A calls resulting in an ambulance arriving at the scene has increased by 7.4% year on year, from 2.5m in 2011/12 to 3.3m in 2015/16.
This demand is rising at a faster rate than emergency admissions to hospital and A&E attendances.
Richard Webber, national spokesman for the College of Paramedics, said in the report: "While we are sympathetic to hospitals that are forced to implement diverts so that they don't reach dangerous levels of overcrowding, in non-urban areas in particular the extra time taken to reach more distant A&E departments is significant.
"There is a 'double whammy' in that not only do crews have to drive further away once a divert is implemented - once that's happened, an ambulance crew will then also need to travel further to get back to their own area to respond to the next emergency call."
An NHS England spokesman said: "Currently too many ambulances are dispatched to simply hit targets rather than attend to those patients most in need.
"This is why we're carefully testing a change to the way in which the services can respond and will make our recommendations known in the coming weeks."