Introducing regional pay and conditions in the NHS would lead to lower standards of patient care, according to the Royal College of Nursing (RCN).
Scrapping the national agreement on terms and conditions in a bid to save money and jobs is the "wrong solution" to the challenges trusts are facing, the union said.
The RCN issued the warning after 20 trusts in south west England joined a consortium that is considering a regional approach to cut costs.
In a briefing, the RCN said the move would exacerbate inequalities and harm patient care, be bureaucratic and expensive to implement and result in a skills drain with staff moving away from lower paid areas.
The RCN also argued the approach lacked economies of scale which would take money away from patient care.
Dr Peter Carter, RCN general secretary, said: "The cartel in the south west alleges that cutting terms and conditions of staff would save jobs. We say that is simply not true and what would actually happen is a skills drain as staff move away. Any trusts looking at such a draconian cost-cutting exercise should look again and think what this will mean to patient care."
He added: "NHS organisations need to stop labouring under the illusion that regional pay is a panacea to their financial troubles. It is not. This would be a fool's economy. It is the wrong solution to the challenges these trusts are facing."
The RCN said it would be writing to senior managers at the trusts involved in the the South West Pay Terms and Conditions Consortium to raise its concerns.
The group was set up in June and recently published two discussion papers papers outlining financial pressures on the service and potential ways of saving money. The consortium said a more "fit for purpose" system of pay and conditions, which includes longer working hours and cuts to annual leave, sick pay and on-call payments, could save more than 6,000 jobs.
A spokesman for the Department of Health said: "There are no plans to cut NHS pay. Pay agreements need to be fit for purpose." He added: "Trade unions have not reached a national pay agreement with NHS employers over the past 18 months, prompting the South West Consortium to begin open and transparent discussions with staff and local trade unions. No decisions or even formal proposals have yet been made."