Increasing numbers of paramedics are leaving NHS ambulance services, according to figures obtained by the BBC.
Senior staff say remaining paramedic crews are under greater pressure than ever before to meet demand.
At least 1,015 paramedics left their job in 2013-14, compared with 593 in the same period two years earlier.
The Department of Health says it is spending an extra £28m on the ambulance service in England this year.
London Ambulance Service saw 223 paramedics leave in 2013-14, four times the number in 2011-12, and the largest increase in the country.
An internal document, produced by London Ambulance Service and seen by BBC Radio 4's The Report programme, suggests morale among paramedics is low.
It says three-quarters of paramedics surveyed had considered leaving the service in the past 12 months.
Anonymous paramedics quoted in the report point to rising workloads as one of their greatest grievances.
Alison Blakely, a paramedic in London for 10 years, says that while she loves the job, when on shift, she often doesn't even have time for a break.
"You use hospital facilities for toilets as much as possible, and eat and drink as and when you can," she says.
"The control room do try and get us rest breaks, but due to the demand currently [they are] rare."
A London paramedic who wanted to remain anonymous, told the BBC that sometimes there were as many as 200 emergency calls on hold, and that the service "haven't got enough vehicles or staff to cope".
"When I joined the job, it was very unusual for someone to leave the service other than through retirement, but over the last two or three years it's escalated beyond belief," he said.
But this is not just a London problem.
Association of Ambulance Chief Executives chairman Anthony Marsh says a surge in 999 calls this year and higher numbers of paramedics leaving some services, means the remaining front-line staff are facing pressures that are "greater than they've ever been".
"Traditionally, ambulance services receive just over 4% more 999 calls each year, and we have done for the last 10 years - some years a little bit more than that, some a little bit less - but this year we're seeing substantially more 999 calls," says Dr Marsh.
This growth in emergency calls has outpaced the rise in numbers of qualified ambulance staff, which has increased on average by 1.6% each year in England over the past decade.
In 2011-12, there were 13,828 paramedics employed by the 12 of the 13 emergency ambulance trusts in the UK that responded to a request for the data by the BBC. This grew to 15,004 in 2013-14.
Dr Fiona Moore, medical director for London Ambulance Service, estimates there is a shortfall nationally of up to 3,000 paramedics.
And she says expectations of what the service is for have also changed.
"We've seen an increase in calls from the 21- to 30-year-old group, and I think that now reflects the sort of supermarket culture we now have, so if you can buy a loaf of bread at 04:00 in the morning, why can't you access your healthcare when it's convenient to you?" she says.
The trust in London is taking action to try to reduce staff workloads, and improve the service.
It has offered more than 180 paramedics jobs on a recruitment expedition to Australia and New Zealand and has recently increased the number of calls that do not receive an ambulance but are instead referred on to other services.
But nationally, the number of new paramedics recruited in 2013-14 was lower than the year before, and some paramedics are concerned that the number of new recruits coming through degree courses is too low to meet demand.
This year the Department of Health has provided £28m to ambulance trusts to help cope with the extra emergency calls.
This year we're seeing substantially more 999 calls”
Anthony MarshAssociation of Ambulance Chief Executives
Hear more on The Report on Thursday, 9 October at 20:00 BST onBBC Radio 4 or download The Report podcast.