NHS Greater Glasgow and Clyde said it had avoided a serious crisis in August this year after an unexpected fall in the number of middle grade trainee doctors available to staff vital departments, including emergency care, anaesthetics and obstetrics and gynaecology.
However, a report to the health board warned it could face similar problems in 2013 and that a failure to tackle training and recruitment issues now could "lead to services being unsustainable or facing crises".
The problems have been driven by recent changes in the way medics are trained, with the European Working Time Directive implemented in 2009 limiting trainee doctors to working 48 hours a week.
A crucial element of MMC is allocating the right number of trainees to a particular discipline and avoiding a surplus of medics in any one field. However, the health board was told staffing projections had proven inaccurate and the numbers of doctors retiring or leaving certain specialities had been underestimated.
The report added that "despite repeated advertisements, recruitment to staff-grade posts had been very poor and retention was also poor among those who had been appointed".
Dr Jennifer Armstrong, a medical director to the board, said: "I remember as a junior the norm was that you clocked in on a Friday evening and worked straight through until Monday morning, snatching sleep where you could. Among my cohort it wouldn't have been unusual to work 100 hours a week.
"After the European Working Time Directive was introduced we now need around two to four junior doctors to cover a slot that once might have been filled by one or two, and at the same time there are separate regulations we have to follow on not creating an over-supply of specialists in certain areas. So, we're seeing a reduction in junior doctors at a time when we're moving towards 24/7 working, and the two of those things together could create a perfect storm."
She added that the health service was juggling an ageing workforce at a time when the number of junior doctors entering the profession was being reduced. There were 69 fewer junior doctor posts in Scotland in August this year compared to 2011.
The problems around filling junior and middle grade posts have been echoed by other health boards, including NHS Lanarkshire, which previously reported shortages in elderly care, anaesthetics, neonatal and orthopaedic care.