Up to 300 frontline staff at NHS Direct could lose their jobs as the service tries to cut costs, the Royal College of Nursing (RCN) said today.
New shift arrangements are being brought in as the new 111 number for non-emergency services is phased in, the union said.
Workers who are unable to join the new shift pattern will lose their jobs, including those with flexible working arrangements already in place, such as those caring for children or working less than 15 hours a week.
The RCN also expressed fears for the future of disabled staff working at the service.
Those affected will have the option to reapply for any remaining shifts.
The RCN said it and other unions had been consulted on the changes and had opposed them.
In the short-term, 111 will run alongside existing local telephone services and NHS Direct but in the long-term it will become the single number for non-emergency care.
RCN chief executive Dr Peter Carter said patients value the service, which could cut referrals to GPs and unnecessary trips to hospital.
"The evidence suggests that this expert advice has kept one and a half million people out of A&E, and saved the NHS £213 million a year.
"Our fear is that patients, who can often be extremely worried or distressed, will receive a stripped back service from NHS 111, with more being advised to dial 999 or go to A&E, which is far more costly.
"At a time when the NHS as a whole is under pressure to make savings, it seems nonsensical that one part of the health service is in effect picking up the tab for another.
"We know that NHS Direct is in a difficult position at the moment, however many of the staff have worked there for years and are naturally very worried and upset about the future.
"NHS Direct also employs higher numbers of disabled workers who may not be able to cope with the physical demands of a hospital ward, yet are still able to provide sound clinical advice to patients.
"If these workers lose their jobs they may struggle to find future employment within a healthcare setting and as a result the NHS will lose their expert skills."
Dr Carter said NHS Direct is not the only organisation tendering to deliver NHS 111 services.
The new system will allow private and GP out-of-hours providers and the ambulance service, among others, to deliver the service.
"Not only will this fragmentation lead to yet more postcode lotteries across the country, we will also lose the highly beneficial national picture that NHS Direct statistics provide," he said.
"I would urge the board of NHS Direct to take the time to think through this process and while they may be under pressure to make savings, there could be other means of doing so.
"Equally, the Government would do well to look at the long-term cost effectiveness of NHS Direct and the service it provides, rather than attempting to steam roll in a cheaper alternative that compromises patient care."
NHS Direct said 79% of frontline staff had been offered "one of their preferred roster patterns".
Some of the more popular options have been oversubscribed, which means about 20% did not get their preferred choice.
Tricia Hamilton, chief nurse at NHS Direct, said: "The RCN is wrong to suggest that 300 frontline staff could lose their jobs because of our plans to introduce new roster arrangements.
"There are enough spaces for everyone to be allocated to a new roster pattern.
"The driving force behind the plans has always been to ensure our staff are available when our patients need them. This change is not driven by a need to make cost savings.
"We have seen a change in the profile of our calls since the introduction of NHS 111.
"Early evidence from the pilots shows that this will be a more out-of-hours and weekend service than the 0845 service since it will be the front door to all GP out of hours services.
"At the moment, because of some long-term issues with rostering, the organisation is not able to guarantee that it can meet this changing demand effectively."