The establishment of centralised specialist centres could pose a risk to patients and downgrade local hospitals in London, said the British Medical Association (BMA).
Proposals from Healthcare for London will encourage patients to visit the specialist centres instead of local hospitals, which could financially impact them.
It said that a lack of nocturnal surgical cover for a few services at the centres could lead to patients being transferred in the night, posing a risk to their safety.
It said that specialist centres are suitable for only some problems and may not be the right place for cases treated in A&E departments.
It asked for more evidence and detailed costing of the proposal before giving it the go-ahead.