NHS Direct is to close most of its call centres with the loss of hundreds of nursing and other jobs, union leaders claim.
Unison said 24 out of 30 sites will close at the service, which employs 1,500 frontline nursing and other professional staff.
It follows government plans to replace the nurse-staffed NHS Direct line with a new service, in which people without such extensive medical training will answer calls, in a bid to save £20m a year.
Sandra Maxwell, Unison's convenor at NHS Direct, said: "Hundreds of dedicated nursing and NHS professionals are to be made redundant at a huge cost, when their skills could be used within the new NHS111 service if only the Department of Health took some decisive action."
National officer Michael Walker said: "The Secretary of State for Health should step in and stop this disaster immediately. We need action and we need it now.
"Axing dedicated hard-working nurses is never a good idea at any time, but this will directly impact on patient care.
"There is no doubt that patients will suffer as a result of this move."
Unison said large centres in Bristol, Sheffield, Wakefield, Nottingham, Hull, Stafford, Chelmsford and Newcastle were among those set to close.
Fears for staff
"Given that many of these NHS call centres are in areas of high unemployment there are very real fears for the staff's future job prospects.
"The government spending millions on making nurses and NHS professionals redundant at a time when the NHS is under great strain is truly scandalous," said a union spokesman.
An NHS Direct spokeswoman said the company had been bidding for contracts across England to provide the NHS 111 service, which is used when a patient needs medical help but it is not a 999 emergency.
The organisation has been awarded contracts to provide the NHS 111 services for 34% of the population in England.
Six sites will remain
She said the 111 service will be delivered from six of its 30 call centres, which are: Middlebrook, near Manchester; Carlisle; Dudley; Exeter; Milton Keynes and London.
"This does not necessarily mean we will be closing the other sites," she said.
The organisation is waiting for some decisions by the NHS Commissioning Board before the future of the other centres will be decided, she added.
Every year, NHS Direct answers 4.2 million calls for health advice and information for patients in England.
Healthcare professionals, nurses, and dental advisers give expert information and advice over the phone and are available 365 days a year.