As part of the changes, crews will be "booked off duty" during meal breaks and unable to attend call-outs during that time.

Unison said the changes would result in an "inferior service" for the public.

The Westcountry Ambulance Services NHS Trust said patients would still receive a "quality emergency service".

'Willing to provide'

Currently crews continue to be available for calls if they come in during a break.

But Unison said if that change is introduced, control room staff would not contact a crew during a break, even if someone collapsed outside an ambulance station.

Ian Hopkins, of the union, said: "Our members are willing to provide a service during their meal breaks.

"That way we would have the right crews in the right place with the right skills and experience. This can be the only safe way to provide cover.

"We do not believe that a reduced role is an appropriate professional response from the ambulance service."

Last month, the BBC revealed that many ambulance stations will face a cut in 24-hour cover, with rota patterns for ambulance technicians and paramedics changed from 12-hour to 10-hour shifts.

Life-threatening situations

That is partly because of the cost of introducing Agenda for Change within the NHS, which was supposed to mean better pay for ambulance staff.

The Westcountry Ambulance Services NHS Trust faces a shortfall of �4.5m.

But it said it was investing about �7m in ambulance staff pay in the region over the coming year.

A spokesman said all the proposals provided more ambulance resources to respond to emergency calls and was aimed at getting the right treatment to patients at the right time and in the right place.

He said: "The trust is confident that patients in need of an ambulance for life-threatening situations will continue to receive a quality emergency service from professional and dedicated staff."