Doctors are struggling to cope with a "recruitment crisis" which is making government plans for seven-day practices "unrealistic", GPs have warned.
More than one in 10 GP roles in England is vacant, according to the Royal College of General Practitioners (RCGP), which is calling on the Government to concentrate its efforts on strengthening existing services rather than stretching current resources further.
Ministers have promised seven day access to GPs by 2020, and have pledged 5,000 extra doctors to help achieve that.
Dr Maureen Baker, chairwoman of the RCGP, said: "We are in dire straits if we do not act to address the GP recruitment crisis immediately and ensure that there are enough GPs in the system so that practices do not have to run with a substantial number of vacant positions.
"Our new research brings home how difficult GP practices are finding it to recruit new doctors and retain existing ones. It is simply unrealistic to be thinking about seven-day working when our existing five-day service and out of hours GP services are under so much pressure."
The RCGP, which surveyed 549 practices, found 10.2% of full-time equivalent positions are vacant, 61% of which are currently filled by locum or agency staff.
More than two thirds of practices (64%) reported finding enough locum doctors to meet patient demand "difficult" or "very difficult".
Dr Baker said the Government should focus on improving the five-day GP service and publicise out of hours services better, describing plans for family doctors to open for longer hours as "pie-in-the-sky".
She said: "The Government needs to move away from its obsession with 'providing a seven day NHS' and do more to implement the joint 10-point plan to build the GP workforce and 'recruit, retain and return' thousands more GPs as soon as possible, so that we can provide a good, solid and safe five-day service, and out of hours service, for our patients.
"Routine seven-day working may improve patient safety in hospitals but in general practice it could have the opposite effect."
A Department of Health spokesman said: "Patients want to be able to see a GP in the evening and at weekends.
"There are already 2,500 practices working towards this and they understand we're not asking individual GPs to work seven days a week - it's about surgeries working together to provide this service."