A leading health think tank on Thursday appealed to the government not to impose "polyclinics" on the National Health Service - saying they should go ahead only where the benefits and costs are clear.

Lord Darzi, the health minister, has recommended that London set up 150 such super surgeries offering X-ray and other diagnostics, blood tests and minor surgery as well as more traditional family doctor services.

The concept, however, is ill-defined and a number of other models exist - including a "virtual polyclinic" in which some GP surgeries specialise in some services for other GPs.

But the call for London - and pressure from the health department for other parts of the country to build them - has triggered a political campaign against them by the Conservatives, backed by the British Medical Association, which has spent large sums leafleting homes and placing advertisements claiming that huge numbers of local GP practices are at risk.

On Thursday the King's Fund health think tank tried to bring some rationality to the debate. It said rapid access to diagnostic services would bring benefits for some patients. Polyclinics could help redesign services around the needs of patients and deliver more integrated care, particularly for people with long term conditions.

It added however that the evidence from other countries is that often those opportunities do not get realised. And while some patients would get a more convenient service, the large scale super surgery could make it much more difficult to visit a family doctor in rural areas.

Changing technology and patient expectations meant it was right to review the way care is provided, Niall Dickson, the King's Fund chief executive said. "But we appeal to ministers to make it abundantly clear that there will no compulsion on local NHS organisations to erect buildings or follow this or that centrally dicated model of care," he said. "Polyclinics may be the right answer in some areas. They will not be right for others."

There is "substantial evidence" that shifting care out of hospital costs money rather than saving it, the report says. And with few specialists currently based in the community, there will be "substantial" workforce challenges in getting them up and running.