Four senior GPs at a practice in Essex are quitting claiming cuts in NHS funding mean they will not be able to maintain the current level of service.
The four partners at Sutherland Lodge Surgery in Chelmsford claim doctor to patient ratios will more than double.
A patients' representative at the surgery said "what you're doing is spreading that money so thinly, it won't have any good effect."
NHS England said the change made GP funding more "equitable" in Mid Essex.
The four senior partners - Dr Ged Cunniffe, Dr Rob Climie, Dr Joanna Birn-Jeffery and Dr Hilary Ramsay - have written to patients saying they will lose a third of their current funding over two years.
"This loss would mean a severe cut in the services we have worked so hard over the years to develop," the letter said.
"We feel very strongly that after the inevitable rise in the doctor to patient ratio, which will move from 1:1,890 to 1:4,000, we will not be able to continue to provide the current level of services nor be comfortable in providing the significantly reduced level of service that the new contract would require."
Robert Good, from the surgery's Patients' Representative Group, said: "There seems to have been a change where funding is to be removed from this and another five surgeries and spread out among the other surgeries in Mid Essex.
"What you're doing is spreading that money so thinly, it won't have any good effect."
NHS England said a review had been carried out to ensure "GP funding was equitable across all Mid Essex's 47 practices" and it would work to appoint a new provider for Sutherland Lodge Surgery's services from 1 July.