While polyclinics may be able to offer patients a wider range of services, there is no evidence that larger primary care trusts deliver better care than smaller ones, according to a new report.
A major analysis of the government's plans for "super surgeries" found that they will not ease pressure on NHS hospitals and patients will be forced to travel further while seeing no improvement in their care.
The study, from the influential think-tank the King's Fund, will come as a blow to ministers, who are already facing mounting opposition over polyclinics.
The British Medical Association (BMA) has launched a Support Your Surgery campaign asking members of the public to sign a petition against private involvement in the NHS and expressing fears over the closure of local GP practices.
The Conservatives released figures in April suggesting that up to a fifth of GP surgeries in England were under threat. But Health Secretary Alan Johnson has hit back, saying the 150 new "GP-run" health centres being opened are being paid for with new money.
He has insisted the plans will not lead to a reduction in traditional GP services.
"We are opposed to polyclinics. We pride ourselves on giving a personal service and knowing our patients well and this is essential for continuity of care. All this will be lost with polyclinics." - Louise Robinson, Statham Grove Surgery