BBC London has learnt up to six Accident and Emergency (A&E) centres may be threatened with closure.
A&E services at Chase Farm Hospital, in Enfield and Queens Mary's Hospital in Sidcup, are listed for closure.
The closure of the A&E department at King George Hospital in Ilford, Essex, is also being consulted upon.
And concerns have been raised by MPs in south west London about the future of A&E at Kingston Hospital. But the NHS says "no firm plans" are in place.
Lib Dem MPs Susan Kramer and Edward Davey said they have been told by five senior NHS staff, including four chief executives and one senior clinician, that there is a proposal to close the A&E ward at Kingston.
London Health Emergency, which campaigns against NHS spending cuts, has told BBC London it believes at least one A&E centre in West London is also under threat.
Meanwhile, BBC London reporter Karl Mercer said: "Whittington Hospital may have to merge its A&E with the Royal Free Hospital, according to NHS plans which are going out for consultation."
But an NHS London spokesman said: "There are no firm plans at present to reconfigure other services, including A&E at the Whittington.
"Any proposals would first be put before Londoners for consultation.
"We do not recognise the figure that suggests six A&Es in London will close."
Campaigners have said the alleged cuts are part of a massive reform of the capital's healthcare, which sees many A&Es replaced with other types of clinics, such as polyclinics.
NHS London has revealed plans for more than 100 polyclinics across London over the next five years, which will offer a wider range of services in one place.
But an NHS London spokesman insisted: "Polyclinics will not replace A&Es.
"They are being introduced to make it quicker and easier for Londoners to access to the care they need.
"Polyclinics will also support A&E staff to care for those who need care for serious and life-threatening emergencies."