Prime Minister Gordon Brown has denied the government plans the mass closure of district general hospitals.
Health bosses across London are planning a shake-up
He was challenged on plans for reform of the NHS in London put forward by new Health Minister Sir Ara Darzi, a practising surgeon.
Sir Ara Darzi proposes 150 new polyclinics - super GP surgeries - in the capital, alongside more specialist hospital units.
Critics believe this would put traditional hospitals under threat.
Although Sir Ara's report focuses solely on London, he has also been placed in charge of a fundamental review of the NHS across England.
Speaking at prime ministers questions, Mr Brown insisted there were no hospital closures planned and accused Conservative leader David Cameron of running a "scare campaign".
But Mr Cameron said Sir Ara was on record as saying the days of the district general hospital were over, and what was needed were fewer, more specialised units.
He said: "What can that mean if it does not mean cutbacks in departments, and closures within existing hospitals?"
Sir Ara was asked by NHS London, the body responsible for overseeing health trusts in the capital, to carry out a review on the future of healthcare in London.
It will act as a guide to London health chiefs who are planning a shake up in services as part of a wider reconfiguration of major hospitals across the country.
London currently has 32 acute trusts - some operating hospitals from several sites - but Sir Ara has recommended complex and specialist care be carried out in fewer hospitals.
He said some of the hospitals left could pick up non-emergency operations and other non-urgent care.
However, these would also face competition from polyclinics under Sir Ara's plan.
The clinics would incorporate a GP service alongside diagnostic tests, social care, mental health services and some minor surgery.
They would be equipped with x-ray and ultrasound machines, allowing consultants to run services closer to patients' homes.
The review said that by 2017 a network of polyclinics throughout London could provide up to 50% of outpatient treatment currently carried out in hospitals.
He also said more care should be carried out at home, including rehabilitation and chemotherapy.
Sir Ara said the blueprint would provide better care as evidence suggests specialist centres for serious conditions - such as strokes - save more lives than traditional district general hospital models.
And he estimated it would save an estimated £1.5bn a year as it is cheaper to provide care out of hospitals.
But Geoff Martin, of the union-funded pressure group Health Emergency, said it would pave the way for the closure or downgrading of nine hospitals.
"This review of services by NHS London is a package of cash-led cuts to local services dressed up as a rational planning process."
Sir Ara Darzi believes people should get treatment locally
But doctors are also warning the review could have wider ramifications.
The national review Sir Ara has been recently asked to conduct was hailed as a "once in a generation" opportunity.
He has been given a wide-ranging remit, but some fear it could be used as a basis to justify cuts.
Jonathan Fielden, chairman of the British Medical Association's consultants committee, said: "It is pushing an old agenda, not a new one. Our concern is that this strips resources from hospitals and GP services.
"Clinicians have not been as involved as they should have been with this review and the concern [with the national review] is that that will continue.
"The last administration worked by having a pre-conceived plan and then having a consultation as a sop to the public.
"The fear is this could happen again. Changes to hospitals must involve clinicians and the public."
A Department of Health spokesman said: "It would be wrong to assume the solutions for London will be right for the rest of the country."