Doctors providing out-of-hours care are being replaced with less qualified staff such as nurses in some areas, a poll suggests.
Many GPs have opted out of evening and weekend work
Pulse magazine found 19 out of 50 primary care organisations it surveyed had replaced doctors with cheaper alternatives in some areas.
The introduction of new arrangements for out-of-hours care was recently criticised by MPs.
The Department of Health said most patients benefit from the new system.
The Pulse survey found another 10 organisations were considering cuts to the number of doctors they employed out-of-hours.
It found some organisations had cut doctor numbers by as much as 50%.
Instead, they were increasingly relying on nurses, and emergency care practitioners, who are trained in paramedic skills, to provide the service.
The House of Commons Public Accounts Committee last month described the handover of responsibility for out-of-hours care from doctors to primary care trusts in 2004 as "shambolic".
The average GP lost £6,000 a year for opting out of providing evening and weekend cover - and many decided this was a price worth paying.
PCTs now arrange the cover, using in-house teams, GP co-operatives and private companies.
Dr John Loveday, a GP in Chesterfield, Derbyshire, told Pulse local managers were trying to provide a service with inadequate resources.
He said: "Many nurses are not specially trained in handling medical emergencies. I think it probably will damage the quality of care."
Paying a price
Katherine Murphy, of the Patients Association, said the changes were being driven by poor financial management, rather than good patient care.
"Patients have had no say in these changes and do not understand the new titles of staff who may be treating them.
"They are paying an increasingly heavy price for a clumsily negotiated contract."
Dr Laurence Buckman, deputy chairman of the British Medical Association's GPs committee, accepted that doctors still had a key role to play out-of-hours.
He said: "While other clinicians, such as emergency care practitioners, have a valuable role in providing out-of-hours care, there will be times when only a doctor will do."
A Department of Health spokesman said: "Most patients are benefiting from improvements in out-of-hours services thanks to the new arrangements.
"PCTs have a responsibility to ensure they provide, or secure provision of, a high-quality, sustainable service for their local population.
"All out-of-hours services must be delivered to the national quality requirements.
"Only last month we published a toolkit which will help PCTs and out-of-hours providers to undertake routine checks to monitor and continually improve the quality of their services."