Up to 1,500 NHS Direct staff are at risk of redundancy, the Royal College of Nursing (RCN) has said.
The RCN said that a consultation, launched with trade unions on Monday, revealed that 1,482 whole-time equivalent staff are at risk. But NHS Direct said that positions would only be officially put "at risk" at the end of the consultation in March.
A spokeswoman said that the Department of Health has agreed that NHS Direct's frontline staff will be offered the opportunity to move into a job providing the new NHS 111 service.
While NHS Direct will take over 111 duties in some regions, other providers such as ambulance trusts and GP out-of-hours organisations will be running the non-emergency helpline in other areas.
The move could lead to the closure of a number of NHS Direct's 30 sites in England. But the spokeswoman added that the number of posts likely to be at risk does not equate to the number of staff who will be made redundant.
Dr Peter Carter, chief executive and general secretary of the RCN, said he was "extremely concerned" about the future of the service.
He said: "For a long time we have been warning the Government that plans to replace NHS Direct with the 111 service will see a loss of expertise and lead to more people having to go to hospital for medical advice.
"These plans amount to abolishing a nurse-led self care and advice service for cost-cutting reasons, but this will prove to be a false economy when it increases the pressure on ambulance services, A&E wards and GP surgeries."
He added: "We remain extremely concerned about the future of the service as it will not be possible to provide the same level of care and advice with such a large reduction in experienced, specialist staff. We will continue to work with other unions and NHS Direct to minimise the number of redundancies."
Nick Chapman, NHS Direct chief executive, said: "NHS Direct is committed to avoiding redundancies wherever possible by redeploying staff into suitable alternative employment providing the new NHS 111 service with NHS Direct or one of the other providers of the service, or into roles in the wider NHS."