Health bosses have criticised the legal system which allows doctors from the European Union to work in the UK without taking either a language or competence test.
The independent regulator for UK doctors, the General Medical Council (GMC), said the system was "profoundly unsatisfactory".
The GMC maintains a register of all doctors eligible to practise medicine in the UK.
A spokeswoman for the GMC said all doctors were checked to confirm their qualifications, training and certificates of good standing.
She said doctors who qualified in the UK, regardless of their nationality, were entitled to be registered provided their fitness to practise is not impaired.
Those who qualified outside the EU can be asked to take an English test and to demonstrate their knowledge and skills to a board before they are eligible to be registered.
Doctors from a European state are eligible for the register, provided their fitness to practise is not impaired, but by law, the GMC may not test the medical skills, knowledge and language proficiency at the point of registration.
"The current situation is profoundly unsatisfactory," said Niall Dickson, chief executive of the GMC.
"We are not able to give the level of assurance we want because of the present legal framework.
"Our position is clear. In order to ensure patient safety, when doctors first register with us, we need to be able to test their English language proficiency and we need to be able to test their clinical knowledge and skills.
"At present we can do that for doctors from outside Europe but we can't do it with doctors from within the European Economic Area.
"We will continue to work with the Department of Health and through the European Parliament to secure the changes we need. The main obstacle to progress is the complex legal framework."
The GMC spokeswoman said there are 218,692 doctors on the UK register who hold a right to practise. She said out of those 139,080 gained their qualification in the UK.
She said that although the GMC supports free movement of doctors in the EU, the regulator would like the discretion to test the English language proficiency and clinical knowledge and skills of all potential registrants to safeguard patients.