Urgent checks are to be carried out on around 3,000 foreign doctors after a fake psychologist was able to work for 22 years without a qualification.
Zholia Alemi claimed she had a primary medical qualification when she first registered in the UK in 1995.
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In reality, her claim to have a degree from the University of Auckland in New Zealand was fraudulent.
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Alemi was later jailed for fraud in October this year after she took advantage of an elderly patient, changing her will to make herself a beneficiary.
The General Medical Council has apologised for ‘inadequate’ checks made in the 1990s and for ‘any risk arising to patients as a result’.
Alemi's deception has now triggered an urgent investigation and checks on the licences of potentially thousands of other doctors.
While working as a consultant psychiatrist for a dementia service in West Cumbria, Alemi exploited her relationship with her vulnerable victim and applied for power of attorney in her name.
She denied charges of fraud and theft but was found guilty and sentenced to five years in prison following a trial at Carlisle Crown Court.
After her sentencing an officer from Cumbria Police described her crimes as ‘abhorrent’.
In a statement, the GMC said that Alemi joined the medical register in the UK under a section of the Medical Act which has not been in force since 2003.
It allowed graduates of medical schools in certain Commonwealth countries, including New Zealand, to obtain registration on the basis of their qualification, without having to sit and pass the standard two-part assessment of their medical knowledge and skills - the Professional and Linguistic Assessment Board exam (PLAB).
These documents were not subject to the rigorous checks that exist today, the GMC has now admitted.
The records of up to 3,000 still licensed doctors who registered via the same route are now being urgently reviewed.
Charlie Massey, chief executive of the GMC, said: "These are serious issues and we are investigating them urgently.
"It is extremely concerning that a person used a fraudulent qualification to join the register and we are working to understand how this happened.
"We have brought this to the attention of police and other agencies, including NHS England, so that they may also take any necessary action to support patients and answer any questions they may have.
"Our processes are far stronger now, with rigorous testing in place to ensure those joining the register are fit to work in the UK.
"It is clear that in this case the steps taken in the 1990s were inadequate and we apologise for any risk arising to patients as a result.
"We are confident that, 23 years on, our systems are robust and would identify any fraudulent attempt to join the medical register.
"Patients deserve good care from appropriately qualified professionals and place a great deal of trust in doctors.
"To exploit that trust and the respected name of the profession is abhorrent.'
A Department of Health and Social Care spokeswoman said: "As the organisation responsible for regulating doctors, we expect the GMC to investigate how this criminal was able to register as a doctor and put measures in place to make sure it can't happen again."