A meeting of Clinical Commissioning Groups (CCGs) for Calderdale and Greater Huddersfield agreed on Thursday to centralise services.
All emergency care could be provided at Calderdale Royal Hospital in Halifax, which according to health managers will improve care and save £18m a year.
But the opposing Hands Off HRI group has said it could mount a legal challenge in the future.
Helen Kingston, who is a member of the group’s steering committee, said: “The general consensus is that we are absolutely appalled and that we feel the CCGs were very condescending in their interpretation of the public’s response – basically making out we don’t understand and we were too emotive about the reasons why we don’t want it to go.
“We expect this to go on to a full-on legal battle and judicial review.”
Campaign secretary Nicola Jowett said: “The overall feeling at the moment is that the gloves are off and we will do anything to fight the proposals.
“These people are doctors. They are supposed to care for us and they’ve betrayed us.”
The group will host a meeting at the Methodist Mission on Lord Street, Huddersfield, on Tuesday from 7pm. A second meeting, where an Irwin Mitchell solicitor will be present to discuss plans, is scheduled for November 21.
A spokeswoman for NHS Greater Huddersfield CCG said: “The CCGs have not taken a condescending view of peoples’ concerns. We have reflected on what has emerged from the published report of findings and response to the consultation from Calderdale and Kirklees Joint Health Overview and Scrutiny Committee. This shows that people want a better understanding of the changes. The CCGs will keep people informed as we proceed to the next important stage of developing our Full Business Case.”