Research by University College London, published by the British Medical Journal, found a "performance gap" between international medical graduates and UK graduates.
It urged the General Medical Council, which commissioned the study, to set pass marks "considerably higher" for entry exams taken by international doctors .
It warned that "very few" current candidates would have qualified to work in the UK if the grades were raised in line with the standard of UK graduates.
Around 1,300 foreign physicians are licensed each year by the General Medical Council after passing an exam which assesses clinical and language skills.
Chris McManus, professor of psychology and medical education at UCL, said the performance gap was highlighted by the number of foreign doctors being refereed to the GMC.
He said: "There is no real mechanism for checking that doctors coming from outside Britain have been trained to the same level as British doctors. We wanted to find out what level overseas doctors would have to reach if they were to be as competent as British graduates. I think it's inevitable that the bar will need to be set higher.
"The fact that you already have overseas doctors being over-represented at GMC hearings is indicative of the problem. Many are simply not trained to the same standards.
"It may be that some overseas doctors have had poor training and when they come to Britain they will catch up quickly and thrive in a better environment.
"But alternatively some may feel completely overwhelmed, particularly with new technology that they have not yet come across. And that is of concern."
The GMC asked UCL to carry out the research after it set up a working party to review whether the competency exam needed to be updated.
Figures from 2012 showed that of 669 doctors who were struck off or suspended in the previous five years, 420 had trained abroad.
The findings come as tougher language checks for European doctors come into force this summer.
Niall Dickson, chief executive of the GMC, said: "We are determined to do what we can to maintain high standards of medical practice in the UK, regardless of where doctors receive their training.
"That is why we are reviewing the way in which we assess the knowledge and skills of those seeking to practise here. This review, along with our decision to increase the score we require in our assessment of English language skills, will help us ensure that high standards of practice are maintained."