Medical authorities have begun urgent checks on around 3,000 foreign doctors after it was revealed a fake psychiatrist was allowed to practise for 22 years with no qualification.
Zholia Alemi claimed she had a primary medical qualification when she first registered in the UK in 1995.
In reality, her claim to have a degree from the University of Auckland in New Zealand was fraudulent.
The truth was exposed when Alemi was jailed for fraud in October this year after taking advantage of an elderly patient, changing her will to make herself a beneficiary.
The General Medical Council (GMC) 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.
A special page has been set up by the NHS for people who believe they have been treated by Alemi.
While working as a consultant psychiatrist for a dementia service in West Cumbria, Alemi exploited her relationship with her vulnerable victim, even applying 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.
Alemi’s contract of employment with Cumbria NHS Trust was terminated after her initial arrest in 2016 and she was interim suspended in June 2017 by the Medical Practitioners Tribunal Service.
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."
He added: "Our processes are far stronger now, with rigorous testing in place to ensure those joining the register are fit to work in the UK."