The safety of hospital patients is being put at risk because inexperienced young doctors are too often being left in charge of A&E and other units, Britain’s medical regulator has warned.

Understaffing and pressures on senior doctors mean trainee medics are being forced to “fend for themselves” by looking after patients whose conditions they are not qualified to treat, said the General Medical Council, which has shared the findings of an in-depth study with the Guardian.

“We are very worried when trainees tell us that they have been put in a position where they have been asked to act above and beyond their clinical competence and capabilities. That’s a concern because it creates very clear risks to patients from doctors who may not know what they’re doing,” said the GMC’s chief executive, Charlie Massey.

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“In some cases trainees are being left to fend for themselves when they clearly don’t have the competence to make the decisions about patients’ treatment that are being asked of them. That’s a real concern. Significant proportion of trainees, especially those early in their careers, feel very exposed in terms of what they are being asked to do,” added Massey.

“Trainees left in this situation feel agitated and anxious that they don’t have enough supervision and worried that they might inadvertently cause harm to patients,” he added.

The boss of the regulator warned that junior doctors who act beyond their competence “are at risk of losing their licence” if the care they give a patient proves to be substandard and damages the patient’s health.

In its annual survey of the education and training received by Britain’s 55,000 junior doctors, the GMC heard about incidents that caused it concern about them being left in charge, despite their inexperience.

In one hospital a group of young foundation-year doctors not long out of medical school began an overnight rotation in the A&E unit despite having had no induction or assessment of their skills. “There was a very real risk that they could be working beyond their competence because they were on a night shift with limited supervision, therefore they might have had to undertake tasks for which they weren’t qualified,” said Massey.

In another emergency department a foundation-year doctor on duty in the resuscitation unit called three times for support from senior colleagues but no one came to help her. “She had no choice but to make decisions about that patient herself, even though she wasn’t confident that she was competent to do so.”

Senior medics in a third A&E unit reported that junior doctors were displaying symptoms of post-traumatic stress disorder, such as continually interrupted sleep and uncertain decision-making. “That worried them and that worried us.”

Dr Sarah Hallett, the deputy chair of the British Medical Association’s junior doctors committee, said: “It is an all too common problem that many junior doctors move into new posts without a thorough induction.

“This is not the fault of senior doctors, who we know are under immense pressures and face ever-increasing workloads in a health service that is more and more overstretched. Despite this, it is crucial that junior doctors have access to appropriate training, and are able to work in an environment that is safe for both them and their patients.”

A year on from the junior doctors dispute in England that prompted eight walkouts by trainees, the number rating their training as good or very good remains high at 76%. However, “a worrying number of doctors in training continue to raise concerns about heavy workloads and shortness of sleep while on duty, with some missing educational opportunities because of these pressures”, the GMC’s report says.