Research by University College London, published in 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 1300 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. "It may be that some overseas doctors have had poor training and when they come to Britain they will catch up quickly."