One in five trainee GPs are planning to quit the profession within five years of completing training, according to research.
Leading GPs have expressed concern over the number of family doctors in training who are already considering leaving the profession.
The poll of 310 trainees by the GP magazine Pulse also found that around 14% plan to work abroad instead of working in the UK as GPs.
The Royal College of GPs said the prospect of losing so many training family doctors would be disastrous for the NHS.
Pulse said there have been reports of a number of GP trainees moving abroad following the junior doctors contract dispute last year.
It said a number of respondents to the survey said they were considering leaving the country to work in Australia, New Zealand or Canada.
The magazine said the figures undermine the government's attempts to increase the GP workforce by 5,000 by 2020.
Professor Helen Stokes-Lampard, chairwoman of the Royal College of GPs, said the pressures of the profession are deterring trainees: "This is really concerning and very sad. We cannot - and must not - allow this 'brain drain' of future GPs to be lost to our profession and to future generations of patients.
"We are already having difficulties attracting medical students into GP training, but the prospect of losing new GPs so early in their careers will be disastrous for patient care and the NHS.
"Unfortunately, trainees are seeing at first hand the enormous pressures that today's GPs are facing. Yet, if properly resourced, being a GP can be the best job in the world, with endless career opportunities.
"After a decade of under-investment, we urgently need to turn general practice around and make it the attractive career option that it once was.
Dr Krishna Kasaraneni, of the British Medical Association's GP committee, told the magazine: "Medical graduates want to be GPs so they can treat patients, but increasingly they are struggling to provide even basic care in a climate of rising workload, stagnating budgets and widespread staff shortages.
"Despite constant promises from the Government, there is little sign that the promised extra resources for frontline patient services are emerging."