GP practices are "struggling" to offer enough appointments to meet patient demand, doctors' leaders have warned.
Surgeries are increasingly relying on support from locum doctors as the gap in the family doctor workforce widens, but many practices are having difficulties finding these stand-in medics, the British Medical Association (BMA) said.
A new poll of 2,800 GP practices in England found that 46% frequently have trouble finding locum cover and a further 40% report occasional issues.
The BMA said there is a "recruitment crisis" gripping general practice at the same time as a third of family doctors are preparing for retirement.
Dr Chaand Nagpaul, chairman of the BMA's GP committee, said: "These results show that many GP practices are struggling to find cover to plug the staffing gaps they face and that the vast majority are having to rely routinely on temporary cover.
"The need for locums can be generated by illness, maternity leave or other factors in the workforce and GP locums do an outstanding job of stepping in to provide care to patients at short notice. But increasingly GP practices are facing longer-term vacancies because of the recruitment crisis gripping general practice.
"If a GP locum cannot be found in these situations many practices struggle to offer enough appointments to meet their patients' needs. Last year more than 600 GP trainee places were unfilled, while more than a third of GPs are estimated to be considering retirement in the next five years.
"This comes at a time when many GP practices are buckling under the pressure of rising patient demand, stagnating funding and unresourced work being moved from hospitals into the community.
"In this climate, it is clear there are no longer enough GP locums to cover the widening gaps in the GP workforce. This is undoubtedly adding to the incredible pressure on GP services which has left it in a state of emergency and struggling to provide even basic care to patients.
"The Government needs to begin addressing this crisis and deliver its promised support package for general practice. We need a long-term, well-financed plan to prevent GP services from collapsing."
A Department of Health spokeswoman said: "The new GP contract we agreed with the BMA will include an extra £220 million investment to help ease pressure on the service, and NHS England has committed to increasing investment in general practice by an average of 4.5% every year.
"We are already seeing an increase in the number of GPs being recruited and we are boosting staff numbers with 10,000 new primary care staff by 2020 - helping to deliver a safer NHS for patients seven days a week."