GP surgeries are being put under severe financial pressure because of changes to the way some practices receive funding, leading doctors have said.
The British Medical Association (BMA) said that last year, health officials began to phase out the Minimum Practice Income Guarantee, which the union said was an "important financial lifeline to many GP practices by guaranteeing a minimum level of funding".
A spokesman said that more than 20 GP practices in east London were facing a "funding crisis" because of the changes.
Many other practices around the country were facing similar problems, Dr Chaand Nagpaul, chairman of the BMA's GP committee said.
Dr Nagpaul was joining a protest march in east London today calling for "urgent solutions" to the financial pressures that surgeries face.
"The phasing out of the Minimum Practice Income Guarantee is having a profound and destabilising effect on these important surgeries that provide vital care to patients in some of the most deprived areas of east London," he said.
"Many other practices across the country are facing similar losses in funding at a time when patient demand is rising and other forms of funding are falling.
"These cuts will reduce care and services to patients, and if these practices close then patients could be left without local GP services."
An NHS England (London) spokeswoman said: "We understand the challenges some London GPs are facing as a result of the changes to the Minimum Income Practice Guarantee.
"At the heart of this is ensuring all patients have access to high -quality GP services that are close to home.
"These changes - which are part of a national policy - will help make GP funding more equitable across London, and the majority of practices will gain as a result.
"It means that practices will be paid fairly according to the number of patients they care for and their needs."