Some 3,000 foreign doctors in the UK are being urgently checked after a woman practised psychiatry for 22 years without any qualifications.
New Zealander Zholia Alemi falsely claimed to have a medical degree from Auckland university when she registered in the UK in the 1995.
But she had actually dropped out of medical school in her first year.
The General Medical Council (GMC) - the doctors' watchdog - has apologised for its "inadequate" checks in the 1990s.
The GMC - which decides whether a doctor is qualified to practise in the UK - said it was sorry for "any risk arising to patients as a result" and that it was confident its current processes are "far stronger".
Alemi was jailed for fraud in October after she faked a dementia patient's will in an attempt to inherit her £1.3m estate.
Now, following an investigation by Cumbria newspaper the News and Star, a review has been triggered.
The licences of potentially thousands of doctors are being looked at.
Who is Zholia Alemi?
When registering in the UK in 1995, Alemi claimed to have a medical degree from the University of Auckland - which she did not have.
But her false medical qualification was only discovered after she was convicted of fraud and theft in October 2018 after taking advantage of a vulnerable patient.
Alemi, who was working as a consultant psychiatrist for a dementia service in west Cumbria at the time, redrafted the patient's will and fraudulently applied for power of attorney.
When asked by police whether Alemi had assisted with her financial affairs, the victim said: "I think she just helped herself."
The judge described her crimes as "wicked".
She denied the charges but was found guilty at Carlisle Crown Court and jailed for five years.
Alemi lost her job after being arrested in 2016 and was suspended by the medical tribunal service in June 2017.
How did she become an NHS doctor?
The GMC said Alemi was allowed to join the UK's medical register under a section of the Medical Act which has not been in force since 2003.
The act meant medical school graduates from certain Commonwealth countries - like New Zealand - were allowed to join the register on the basis of the qualification they obtained at home.
They did not have to sit and pass the standard two-part medical test that foreign doctors normally have to pass before they can work in the UK - the Professional and Linguistic Assessment Board exam (PLAB).
What is the GMC doing now?
The GMC said its checks are now more "rigorous" and stronger than they were. The council said that now any similar fraudulent attempts to join the medical register would be identified.
But the records of up to 3,000 doctors - who registered for a licence under the same rules as Alemi - are now being urgently reviewed.
The GMC said it was not expecting to find anything untoward.
Charlie Massey, the GMC's chief executive, called it a "serious" issue and said the police and other agencies including NHS England have been informed.
"We are confident that, 23 years on, our systems are robust and would identify any fraudulent attempt to join the medical register."
The GMC said patients place a great deal of trust in doctors, and "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."
What should concerned patients do?
The GMC has created a web page with advice for anyone who is concerned that they were treated by Alemi.
It urges anyone who was treated by her to contact the GP surgery, hospital or clinic where they received treatment.