More than three quarters of GPs are aged over 55 years old at 762 GP practices across the UK, analysis by the RCGP reveals.
The likely retirement of GPs at these practices leaves 2.5m patients in England alone at risk of seeing their practice close within the next five years, the college estimates - warning that the impact on patient care would be 'catastrophic'.
Around 625 practices in England have more than three quarters GPs aged over 55 - roughly one in 11 practices in the country - along with 71 in Scotland, 37 in Wales and 29 in Northern Ireland.
The college has called for 'drastic action' to tackle the problem, demanding £2.5bn more funding per year by 2020/21 for general practice under the government's long-term NHS plan.
RCGP chair Professor Helen Stokes-Lampard said: 'These new figures paint an extremely bleak picture of the scale of the GP workforce crisis right across the UK.
'GPs will always work their hardest to try to keep practices open, but the harsh reality is that fantastic, caring GPs are burning out, working in conditions that are unsafe for their own health and that of their patients.
'Workload in general practice is escalating, both in volume and complexity, yet the share of the NHS budget general practice receives is less than it was a decade ago – and our workforce is actually decreasing. As a result, many GPs are bringing forward their retirement plans because the pressures they are working under are untenable.'
GPonline reported last month that the full-time equivalent GP workforce had dropped by more than 500 in the three months to June 2018 - and the workforce is down by more than 1,400 since former health secretary Jeremy Hunt set a target in September 2015 to increase the GP workforce by 5,000 by 2020/21.
This website also revealed this month that 1,200 GPs were receiving support from the GP Health Service - which offers help to primary care doctors facing problems including burnout, stress or addiction.
Professor Stokes-Lampard added: 'It is a massive loss to the profession – and patients - to lose our most experienced doctors prematurely when they have huge amounts of knowledge and skill.
'If these GPs do leave, and these practices do close, it will have a catastrophic impact on our profession and the patient care we are able to provide. We have more GPs in training than ever before, but if we have more GPs leaving than entering the profession, we’re fighting a losing battle.'
The RCGP chair urged the government to 'think long and hard' about how to retain GPs and use the £20.5bn funding increase promised for the NHS to ease pressure on general practice.
The five worst-affected CCGs in England were Sandwell and West Birmingham - with 85,105 patients at risk of practice closure, Medway (52,330 patients), Havering (49,761), Ealing (46,909) and Wigan Borough (43,640).
An NHS England spokesperson said: 'These forecasts only stack up if you presume no new doctors or health professionals will start work in general practice in the future. The NHS is spending an extra £2.4bn on general practice by 2020 and training more GPs than ever before.'