A health watchdog representing patients has criticised plans to merge ambulance services across the West Midlands region.
It warned lives would be put at risk if ambulance drivers got lost because of a lack of local knowledge.
Ministers have approved plans to merge four ambulance trusts to create a giant regional service, stretching across Worcestershire, Staffordshire, Shropshire and Herefordshire, as well as the West Midlands conurbation.
Police and fire services are also to merge, creating regional bodies in the same way.
But the Patient and Public Involvement Forum for Coventry and Warwickshire, the official body representing patients, said it was increasingly concerned about the ambulance merger.
The changes would be at the expense of the invaluable local knowledge the ambulance crews and call centre staff have now, said Forum chairman Adrian Edgington.
He said: "Warwickshire is a largely rural area where local knowledge is key. Many houses in small villages do not even have house numbers.
"It is essential for both call centre staff and ambulance crew to understand the local area to be able to respond to emergencies promptly and efficiently.
"Place names are another difficulty for emergency services - in a large organisation will the staff have enough knowledge to distinguish Willenhall in Wolverhampton from Willenhall in Coventry?"
He added: "We as a Forum and the local people we have consulted with are very concerned about the amount of time that will be lost and the lives that could be put at risk if ambulances are going to the wrong destination."
Forum member Jim Leach said: "The proposed merger of the four ambulance trusts will create an organisation that covers an area of almost 6,000 square miles and a population of 5.5 million. This will make it the largest ambulance trust not only in the United Kingdom but in the world.
"The Forum is concerned that the logistics of managing such a huge area will put the organisation under considerable pressure."
A spokeswoman for the West Midlands Strategic Health Authorities said: "There is a real commitment to maintaining local expertise within the new organisation, pending the outcome of the consultation.
"This will be maintained through existing control centres and local delivery units, based on the geography of the current ambulance service.
"The proposals do not include any change to the model of service provision locally or local control centres.
"We don't expect there to be any change in frontline services." ..SUPL: