Almost half of practices that responded to the poll reported that they had GPs who planned to retire or quit UK general practice in the next 12 months.
GP leaders have urged practices to consider dropping unfunded work to ease pressure. Earlier this week, a GPonline survey revealed that one in three practices have stopped providing some services in the past year because they were not being funded by the NHS.
BMA leaders warned that the union's survey findings reveal the extent of the growing GP crisis in England.
A total of 2,830 GP practices - more than a third of all practices in England - responded to the BMA survey, with 294 reporting that they are financially unsustainable.
GPs at 610 practices (one in five) reported that their surgery was in a weak financial position, while just one in 20 reported a strong financial position.
Financial stability for GP practices was worst in London, with 14% of practices that responded warning they were unsustainable and 41% reporting a weak financial position.
The north east of England and the East Anglia had the highest proportions of GPs planning to retire in the coming year, with 42% and 41% respectively.
GPC chairman Dr Chaand Nagpaul said: 'This survey provides further evidence of the state of emergency facing general practice. Almost half of GP practices are looking at the loss of part of their workforce at a time when there is a shortfall in new doctors entering general practice.
'As GP services struggle to replace existing staff who leave, it will inevitably make it more difficult to maintain current services to patients and particularly to offer enough appointments for them. Just as worrying, close to three hundred practices looking after tens of thousands of patients believe their financial future is unsustainable.
'GP practices are facing this dire situation because they are being overwhelmed by rising patient demand, cuts to funding, staff shortages and more unfunded work being moved from hospitals into the community. Given these pressures it is unsurprising that GPs are considering leaving the NHS while new medical graduates are turning their backs on a career as a GP, a situation undoubtedly worsened by the government’s appalling handling of the junior doctor contract.
'With hundreds of GP practices facing financial uncertainty, and close to 300 facing possible closure, we need the government to act urgently to deliver a comprehensive rescue package that safeguards GP services for patients. We cannot have a situation where thousands of patients are left without a local GP practice that can deliver the care they deserve.'
A DH spokeswoman said: 'We know GPs are under pressure and that is why we have agreed record investment for general practice. We saw an increase in the number of GPs recruited last year, and we will continue to boost numbers with an extra 5,000 doctors in general practice by 2020 - helping to deliver a safer NHS for patients seven days a week.'
An NHS England spokeswoman said: 'GP practices may close for a number of reasons, including mergers to create groups that offer a wider scope of services but we have announced a number of measures to help those under pressure, including £10m for struggling practices. While there are over five thousand more full time equivalent GPs than ten years ago, we will soon announce further plans to boost the primary care workforce and ensure general practice continues delivering a high-quality service for patients.'