Cuts to budgets to train doctors could compromise quality of patient care, the British Medical Association has warned.
Many doctors want flexible working
Postgraduate deaneries, responsible for medical training, have seen their bugets fall by £20m from the amount originally planned this year.
The BMA is particularly concerned that this will lead to reduced opportunities for junior doctors to train part-time.
It warns failure to reinstate the funding could cause staff shortages and jeopardise quality of care.
In a briefing paper for MPs, the BMA highlights the case of two deaneries, in London and Newcastle, which are cutting funding for flexible training posts.
These, the BMA says, boost hospital staffing levels and allow junior doctors to train part-time.
Cuts to these posts mean that some doctors are coming under pressure to change their choice of career, the BMA says.
Its research shows that only one in ten young hospital doctors currently works less than full-time - but almost half would like to in future.
Some give up medicine altogether to pursue careers with better work-life balance.
The briefing paper highlights the case of Dr Lauren Williams, a senior house officer in south London.
She is a talented triathlete, but is unlikely to be able to balance her career in medicine with a career in athletics unless she is given the opportunity to train part-time.
After repeatedly being turned down for flexible training in London, she has decided to move out of the capital.
Dr Williams said: "It's common for doctors in my grade to work around 56 hours a week.
"I don't think there's anything wrong with that and I always knew being a doctor would be hard work - I just want to be able to put some time into my triathlon training.
"If I don't get the chance to work part-time while I'm still young enough to pursue my sporting ambitions, I can see myself leaving the NHS altogether."
Dr Jo Hilborne, deputy chair of the BMA's Junior Doctors Committee, said: "Many hospitals already take a dim view of junior doctors who want the opportunity to work part-time.
"What they seem to forget is that it costs £250,000 to train a doctor, and if they then abandon medicine for a career with better work-life balance, there's a huge loss to the NHS.
"Reinstating deaneries' funding would be an excellent way for the government to show it's serious about improving working lives."
A Department of Health spokesperson said this year's NHS training budget had still increased by £290m from the previous year.
"This has enabled us to increase Strategic Health Authorities' training allocations by an average of 7.3%, with every Health Authority receiving 3.2% and many considerably more.
"In addition to these increased allocations an extra £40 million is being made available in-year specifically to train more doctors and dentists.
"How Strategic Health Authorities spend their training budgets is for them to determine in consultation with their Workforce Development Confederations and postgraduate deaneries, in the light of local priorities and national workforce targets."