Hundreds of jobs could go at the telephone advice service NHS Direct as part of a restructuring programme.
NHS Direct was set up eight years ago
Unions warn up to 1,000 positions could be cut, but firm proposals will not be put forward until next month.
NHS Direct said it would try to keep the losses to a minimum as part of a drive to remain in financial balance and respond to demand for web services.
The news comes after NHS trusts have announced 4,000 job cuts in recent weeks in a bid to balance the books.
The health service is in the middle of a financial crisis after running up a £250m deficit in 2004-5 - a figure which is expected to have increased in the financial year which has just finished.
NHS Direct was not in deficit, but a restructuring is needed if the organisation was not to fall into debt in the future.
The 24-hour telephone and internet service was set up eight years ago to provide out-of-hours advice to patients.
Public sector union Unison warned if as many jobs went as feared it would gut the service.
A spokeswoman said: "NHS Direct has had its criticisms, but for a huge number of people it has been a good point of contact for the health service.
"If these cuts go ahead the service would undoubtedly suffer. We hope they won't be as bad."
Interim head of external affairs Ann Grain acknowledged in theory up to 1,000 jobs could go, but said the organisation was aiming to "keep them to a minimum".
"We have to make the organisation fit for purpose. We have 54 call centres and when the website is the growth area we have to ask whether we need them all.
"Eight of the leases are up, but at this stage we do not know whether all will close. It is difficult to say that the numbers the unions are saying won't happen at this stage because we do not have the firm proposals yet. That will come next month.
"However, we are aiming to keep them to a minimum and are likely to recommend increasing the number of front-line staff answering calls. It is scaremongering to say 1,000 jobs are going."