Foreign doctors should face tougher tests of their skills before being allowed to practise in the UK because of concern that too many are not well enough trained, experts urged on Friday in studies published by the British Medical Journal.

The experts, from University College London and Cambridge and Durham universities, recommend raising the pass mark for tests that overseas-trained doctors have to take before working in the NHS in order to ensure that those who succeed are as competent as those who have been to British medical schools.

They highlight what they call a "performance gap" between the two groups, with those who have done their medical degree elsewhere too often behind their British counterparts.

However, the researchers, in two studies published on the BMJ's website, warn that acting on their suggestion could produce a major shortage of doctors in the health service, which is heavily reliant on medics from overseas.

Already 37% of the NHS's medical workforce are from elsewhere, and that trend is increasing.

More than one in four doctors (27%) registered with the General Medical Council (GMC), which regulates doctors, qualified outside the European economic area.

In order to gain clearance to work in Britain such doctors must first pass an English-language test and both parts of the regulator's Professional and Linguistic Assessment Board (PLAB) test.

The PLAB test is intended to ensure foreign-trained doctors have equivalent medical knowledge and clinical skills to those of an NHS-trained doctor who has completed the first year of their foundation level one training.

However, those trained abroad are less likely to perform well in entrance exams for professional bodies such as the royal colleges of physicians and GPs. They are also more likely to be referred to the GMC by someone who believes they are not fully fit to practise safely.

The experts want the pass mark for the PLAB test to be raised to close the gap in performance between foreign and UK-trained medics.

But they concede this could lead to the NHS facing "severe workforce planning challenges", after having seen its reliance on international doctors grow in recent years, including in less popular medical specialities such as psychiatry.

In the first study, the UCL and Cambridge researchers found while the PLAB test is useful for gauging doctors' abilities, those trained overseas did much less well in the royal college exams than those who had attended a British medical school and that their subsequent career progression was not as good.

Raising the PLAB pass mark considerably would exclude some doctors who currently pass it, they say.

But setting it at the right level "is fundamental to ensuring the quality of postgraduate medical education and training, the delivery of medical care of the highest quality, and thus ensuring patient safety in the NHS".

In their study, the Durham researchers found that international doctors who do better in English-language performance tests, score higher in the PLAB and undertake fewer resits – and also tended to have better outcomes in the Royal College of Physicians' entrance exam.

The Department of Health declined to say if it supported the experts' call or not. A spokeswoman said: "Patient safety must be at the centre of NHS care. We are introducing new laws this year, which will allow the General Medical Council to carry out language checks on doctors.

"The GMC has set up a working party to review the exams for foreign doctors to see whether standards need to be raised."