Zholia Alemi, 56, claimed to have a degree from the University of Auckland in New Zealand when she came to work in the UK in 1992.
In reality, the convicted fraudster had flunked her first year and dropped out.
But nobody at the General Medical Council, the watchdog responsible for vetting the background of medics, checked whether her documentation was genuine.
At that time, doctors from certain Commonwealth countries could be cleared to start work simply by presenting their qualifications, without sitting any assessments.
It meant that for more than two decades from 1995, Alemi was free to treat thousands of mental health patients in the NHS, apparently prescribing medicine, making assessments and even sectioning some of the most vulnerable in society.
The deception by Alemi, thought to be of Iranian extraction, was only discovered after she was convicted of trying to fake the will of an elderly woman to steal her £1.3 million fortune.
A judge described her as 'despicable' and jailed her for five years last month.
'If this had been one individual that had slipped through the net it would have been concerning, but the idea that it could be a systemic loophole that has been exploited is hugely alarming,' he said.
'It is understandable that patients are calling for an inquiry – this is of sufficient magnitude that that may well be necessary.'
The GMC admitted its checks had been inadequate and confirmed an 'urgent investigation' was looking into the backgrounds of 3,000 doctors who came to work from Commonwealth countries, before 2003, in the same way.
Incredibly, Alemi had been investigated and given an official warning by the Medical Practitioners Tribunal Service in 2012 after failing to disclose a conviction for careless driving.
She also sectioned psychiatric patients for treatment without the authority to do so, and was banned from working for 12 months as a consequence.