One in four GPs responding to the poll (25%) said their practice had at least one vacancy for a GP partner, and 44% of these vacancies had been unfilled for more than 12 months.
Partnership posts are the hardest to fill, the poll of 271 GPs suggests, with one respondent reporting that her practice had 'given up' trying to fill partnership posts and used locums instead.
But significant numbers of posts for salaried GPs are vacant and many practices report significant, long-term problems finding locums too.
A total of 38% of GPs said their practice had a vacancy for a salaried GP, and 38% of these said a post had been open for more than a year.
Just under a quarter of respondents said their practice had locum posts available and 22% of these had a post that was unfilled for a year or more.
Some practices are struggling with multiple vacancies, reflecting the growing GP workforce crisis. Just over a quarter (26%) of practices with a vacancy for a partner had two or more posts unfilled. Around a third of practices with salaried GP vacancies were looking for two or more recruits.
GPC deputy chairman Dr Richard Vautrey said the findings reflected the 'major recruitment and retention crisis' facing general practice, and warned that GP morale was 'at rock bottom' as unfilled posts left practices struggling to maintain patient safety.
The findings come as part of a series of GPonline investigations into practice sustainability.
Last week, GPonline revealed that one in 10 GPs believe their practice is at risk of closure in the next 12 months because of recruitment, workload or funding issues. The closure of this number of practices could leave more than 5m patients looking for a new GP practice a year from now.
Meanwhile, more than half of GP partners face cutting their take-home pay over the next 12 months to help keep their practice afloat, and many report that they will be considering measures such as reducing use of locum GPs, merging with other practices or increasing skill mix.
One respondent to the poll on vacancies said his practice had 'long-term difficulty getting short-term locum cover'.
Another said recruitment problems had forced a temporary reduction in available appointments. 'We had difficulty covering school summer holidays with locums and had to reduce appointments as a result. We advertised this fact in advance to our patients.'
One GP said her practice received just one application when it advertised a year ago to replace two retiring full-time GP partners, but the applicant 'declined the post and wasn't suitable anyway'. She added: 'We have altered skill mix instead, expanded our nurse practitioner service and [created] a healthcare assistant post.'
One GP said his practice had recruited successfully, but after a long struggle. 'Since June 2016 we have been fully staffed for the first time in six years.'
The GPonline poll findings reflect national data that reveal the workforce crisis facing the profession. Regional variation in the availability of GPs is huge, with England's most underdoctored areas operating with 40% fewer GPs per patient than the best-staffed areas.
Trainee recruitment problems remain too, despite an improvement in total numbers of GP trainee posts filled over the past year, with one in 10 available posts for 2016 vacant.
Dr Vautrey said: 'With the major recruitment and retention crisis, with so many practices carrying vacancies and almost giving up on recruiting GPs, it's no surprise that the workload for those left in the practice is so high and at times puts patients' safety at risk. Few GPs can remember a time as bad as it is at the moment so it's no wonder morale is at rock-bottom.
'Even when practices look for alternative clinicians to provide support they can often struggle to recruit nurses and other healthcare practitioners. Despite the promises made in the GP Forward View most practices have yet to see any tangible help, and when they approach their CCG they are all too often told there is no recurrent funding to support meaningful recruitment strategies.
'Young doctors will only choose general practice in sufficient numbers to turn the current crisis around if they see governments and NHS England putting significant and recurrent funding in to core general practice, something that has to happen urgently.'
An NHS England spokesperson said: ‘We are reversing the historic under investment in general practice with an extra £2.4bn a year by 2020/21 - a 14% real-terms increase - and are on track to deliver more GP recruits than ever before.
'The GP Forward View's initiatives to further boost the workforce are now kicking in and we are supporting local areas to deliver their sustainability and transformation plans, setting out how they will further ensure that strong general practice services remain central to the NHS.’
NHS England officials are likely to reveal this month which practices will receive a share of £16m available in 2016/17 from the Practice Resilience Fund.
NHS England also plans to invest £5m this year to repeat last year's winter indemnity scheme, offering financial support for GPs who take on additional out-of-hours shifts.
It also revealed last week that more than 30 GP practice infrastructure or premises schemes would be the first of 300 practices to receive support from the Estates and Technology Transformation Fund in 2016/17.