The four ambulance services in the West Midlands face the axe under plans to create one regional body.
The West Midlands has four ambulance trusts - Coventry & Warwickshire; Hereford & Worcester; Staffordshire, and West Midlands Ambulance Service, which covers Birmingham, Solihull and the Black Country.
But these will be replaced by a single service under proposals drawn up by local health authorities.
They have been sent to councils, MPs and patient forums as part of a "pre-consultation".
Last night MPs vowed to oppose the scheme, claiming that services will become "less reliable".
But health service union Unison backed the plan, saying it would encourage best practice across the region.
In June, the Department of Health unveiled plans to reform ambulance services across England, including merging trusts. However this was expected to lead to the loss of only one trust within the West Midlands.
Peter Luff ( Con Mid Worcestershire) said he had been surprised when he received a letter from West Midlands South Strategic Health Authority revealing all four would be merged.
He said: "There has been totally inadequate consultation on this proposal.
"I strongly oppose the merger of the ambulance trust, which is a severely retrograde step and must be resisted."
MP Geoffrey Robinson (Lab Coventry North West) said he would raise the plans in Parliament.
He said: "I have serious misgivings about the possibility of a merger of the ambulance services in the West Midlands, which could lead to the establishment of one giant operation."
The MP will meet Malcolm Hazell, chief executive of Coventry & Warwickshire Ambulance Trust, to discuss his concerns on Friday.
But Bronwen Bishop, director of private office at West Midlands South Strategic Health Authority, said a full public consultation would take place before any decisions were made.
She said: "The preferred thinking at the moment is for the four ambulance services to merge.
"But we are not talking about just bringing them together and creating one control room. The services will keep their local identity and there will be several call centres."
Ray Salmon, a West Midland regional organiser with UNISON, said: "We support the proposals.
"There is a lot of good practice in the West Midlands, and we believe that by merging the trusts and sharing best practice, we can have a service that is worthy of the region."
The mergers are part of a major reform of ambulance services, which includes training paramedics to provide more treatment at home, and providing advice from telephone operators to patients who dial 999 with problems which are not emergencies.
Every ambulance trust will also become part of an English ambulance service, instead of operating independently as they do now.
Earlier this year the Government announced plans to close 46 emergency fire control rooms in England and replace them with nine new regional centres.
Under the proposals the West Midlands headquarters at Lancaster Circus in Birmingham will be transferred to a new facility in Wolverhampton.