Some trainee doctors are working beyond their competence due to huge workloads, with many worried about patient safety, according to a new report.
A poll of 55,000 UK doctors in training for the General Medical Council (GMC) found that 43% described their daytime workload as "very heavy" or "heavy".
This rose to 78% of doctors in emergency medicine, with workloads in all areas having got worse over the last five years.
Over half of doctors in training said they regularly work beyond their agreed rota hours, and up to 25% said their working patterns left them sleep-deprived on a weekly basis.
The report also found that doctors with the highest workloads were more likely to report patient safety concerns.
Those who described their workload as very heavy had twice as many concerns about patient safety as those who thought their workload was about right. They were also six times more likely to feel forced to cope with clinical problems beyond their competence on a daily or weekly basis.
The report said: "This has worrying implications for the safety of patients, doctors in training, and public confidence. Our standards are clear that doctors in training should not be expected to find themselves in such a situation."
The poll also found that, when compared with doctors in training who said their daytime workload was about right, those who described their daytime workload as heavy or very heavy were three times more likely to have to leave a teaching session to answer a clinical call once or multiple times each session.
The report said: "While we acknowledge that treatment in busy environments is an occupational inevitability, training time should be protected as much as possible."
Compared to 2014, more than twice as many doctors responding to the GMC survey in 2016 used the opportunity to raise worries about patient safety. Some 838 doctors reported a local patient safety issue in 2016 compared to 404 in 2014.
The GMC warned that many doctors in training are working under such significant and growing pressure that it threatens the training they need to become GPs and consultants.
The GMC also surveyed 23,000 trainers, most of whom said they enjoyed their role, but one in three said they did not have enough time to deliver training.
GMC chief executive Charlie Massey said there was a "tremendous amount" of high quality training across the UK. But he added: "We know the very real pressures our healthcare services are under and appreciate the challenges organisations involved with the training of doctors are facing, but it is vital training is not eroded.
"Those responsible and accountable for the delivery of medical education locally must take appropriate steps to ensure the training of doctors remains protected. Medical training is so often a bellwether for the quality and safety of patient care and patients are directly at risk if support and supervision of doctors in training is inadequate."
A separate poll of 498 junior doctors for the Royal College of Physicians (RCP) found 80% felt their job "sometimes" or "often" caused them excessive stress.
One in four said it had had a serious impact on their mental health and over half said it "sometimes" or "often" had a negative impact on their physical health.
Professor Neena Modi, president of the Royal College of Paediatrics and Child Health (RCPCH), called the results of the survey a "stark warning of the risks to patient safety and care as a consequence of the increasing pressures faced by junior doctors in the NHS".
Dr Pete Campbell, the British Medical Association's junior doctors committee chairman, said: "Patients and the public may be shocked by these findings, but no junior doctor will be surprised. It is still far too common that junior doctors are left sleep-deprived after regularly working beyond their rostered hours, on rotas that are desperately short of doctors.
"We cannot accept a situation where vital training time is being sacrificed in the face of rising pressures on services. This is a short-sighted approach that is already having an impact on the quality of patient care."
Professor Wendy Reid, director of education and quality at Health Education England (HEE), said: "We know that being a junior doctor is challenging and stressful without any additional pressures such as poor rota planning, unsupportive senior colleagues and lack of family time.
"HEE has been leading work to improve the training for junior doctors but this report highlights the importance of the employer in managing the workload, rotas and support at work for doctors in training.
"HEE and NHS Employers have agreed a new Code of Practice which will improve communication and planning of placements and we look forward to working with NHS Employers and the system to improve the working lives of doctors in training."
A Department of Health spokesman said: "We want to support our junior doctors. That's why the NHS has employed 11,900 more doctors since 2010.
"Yesterday, the Health Secretary announced plans to improve junior doctors' training, including more support from consultants, more notice of future placements including where couples are placed, reviewing the appraisals process and investing £10m to bring doctors back up to speed when they take time out to have a family or other caring responsibilities."