Vacancies for GPs are at their highest ever level, research suggests.
A survey of 860 GPs for Pulse magazine found 12.2% of positions are currently vacant.
This is an increase from 11.7% the same time last year, and up from 2.1% in 2011, when Pulse started collecting data.
Some 158 (18%) GPs in the new survey said they had to give up recruiting a GP in the past 12 months after being unsuccessful.
The survey also found that the average time taken to recruit a GP partner has risen from 6.6 months to 7.4 in the last year.
Pulse said some practices are having to resort to hiring non-GPs to fill the gaps, while others have closed down after failing to recruit a GP partner.
A report from the Commons Public Accounts Committee in April found there had been "no progress" in the last year on increasing the number of GPs.
This is despite a Government target to recruit 5,000 more by 2020.
The report said the number had actually fallen, from 34,592 full-time equivalent doctors in September 2015 to 34,495 in September 2016.
MPs said more trainees needed to be recruited, while existing GPs should be encouraged to stay on.
Professor Helen Stokes-Lampard, chairwoman of the Royal College of GPs, said of Pulse's findings: "We know that practices across the country are finding it really difficult to recruit GPs to fill vacant posts, and the degree to which this problem has increased over the last six years is staggering.
"In the most severe cases, not being able to recruit has forced practices to close, and this can be a devastating experience for the patients and staff affected, and the wider NHS.
"Being a GP is intellectually challenging, emotionally rewarding and incredibly varied – it should be an easy profession to recruit to.
"But at present UK general practice does not have sufficient resources to deliver the care and services necessary to meet our patients' changing needs – meaning that GPs and our teams are working under intense pressures, which are simply unsustainable.
"Workload in general practice is escalating – it has increased 16% over the last seven years according to the latest research – yet investment in our service has steadily declined over the last decade and the number of GPs has not risen in step with patient demand.
"This must be addressed as a matter of urgency."
An NHS England spokeswoman said: "This miniature survey of fewer than one in 10 GP practices is statistically incapable of giving an accurate national picture on GP posts, and what's more the survey response rate was even lower than last year which further invalidates any inferences about annual trends."