A doctors' organisation has criticised proposals to replace district general hospitals with new local hospitals in London.
The British Medical Association (BMA) believes the plans would put patient safety at risk and reduce the income of existing hospitals.
Healthcare for London wants to introduce local hospitals to provide emergency and specialist care.
It said the changes would not downgrade existing hospitals.
A spokeswoman for Healthcare for London said a report into the plans "recognises the benefits of taking the most seriously ill patients to world-class specialist centres".
She added: "It also envisages some care currently provided in hospitals being delivered more locally in the community setting."
A pilot scheme and more research was needed before the changes could be considered, the organisation said.
The BMA, which represents doctors across the UK, is concerned the lack of night-time surgery cover at the proposed local hospitals would mean critically ill patients would have to be transferred at night.
Dr Kevin O'Kane, from the BMA, said: "London doctors share the aspiration to improve health services, but we have significant concerns about these proposals.
"They're not supported by enough evidence, they carry clinical risks, and they would leave the average London hospital with a very limited range of services."
Proposals from Healthcare for London say local hospitals would serve around 250,000 Londoners and would provide an accident and emergency department as well as maternity and paediatric units.