Staff at NHS Direct in England are warning they may take industrial action unless bosses change their plans to close call centres and cut jobs.
NHS Direct was set up eight years ago
The nurse-led telephone and internet advice service is trying to save £15m to balance its books by cutting nearly 800 posts and 12 call centres.
But Unison and the Royal College of Nursing have warned staff are angry, saying the proposals would risk lives.
NHS Direct said the proposals would make it more effective and efficient.
NHS Direct was set up in 1997 to help give medical advice to people on how to deal with symptoms and to direct them to the correct place in the event of an emergency. It now deals with 25,000 calls a day.
As well handling millions of calls each year on its helpline, it also displays advice on its website which attracts around one million visits a month.
The plan, which is being consulted on until 16 August, proposes cutting a total of 789 posts, 573 through redundancies and 216 through "natural turnover".
NHS Direct has said that the expansion of some of its call centres would also help create 376 new posts, making the total number actually lost more like 400 out of a total of nearly 3,000.
The 12 call centres due to close are in Doncaster, York, Cambridge, Scunthorpe, Chester, Bolton, Preston, Chorley, Southport, Croydon, Brighton and Kensington.
Any future industrial action is unlikely to include strikes as the RCN committee which governs disputes - just recently reconvened for the first time since 2001 - aims to keep disruption to patient care to a minimum.
Instead, potential measures taken by nurses would probably involve working to rule.
Unison said a decision on future action would be taken once it was known how NHS Direct was proceeding following the consultation.
Dave Prentis, general secretary of Unison, said: "This is a cost-cutting exercise at NHS Direct and could put lives at risk and staff are so angry at the prospect, unions are gearing up for industrial action."
And RCN general secretary Beverly Malone added: "We are calling on NHS Direct to work with us to find a longer-term and thought-through solution that avoids compulsory redundancies and does not compromise the quality of patient care."
But a spokeswoman for NHS Direct said the proposals were designed to "make us more effective and efficient".
She added: "They will help us maintain our excellent reputation for clinical safety and improve public access to health by signposting people to the right NHS service more quickly."
"NHS Direct will study carefully proposals from the unions and other parties in formulating the final changes to be implemented."