A doctor worked at an NHS hospital in llanelli while waiting to stand trial for killing a patient in Spain.
Marcos Hourmann, 51, originally from Argentina, was able to continue working as a surgeon in the UK even after being found guilty of manslaughter.
He even went on to work as a police surgeon in the UK, reportedly earning £10,000 a month, despite his conviction.
Hourmann was able to flout the rules in the UK and lead a double life because of lax European laws. European countries have no legal obligation to alert their neighbours when medcial staff receive convictions.
The Spanish authorities made no such attempt and took the view it was up to Hourmann to tell potential employers about it.
The UK’s General Medical Council (GMC) has now acted to suspend him pending a thorough investigation.
Hourmann carried out a mercy killing on an 82-year-old female patient who, he claimed, had asked to die.
He gave her a lethal potassium chloride injection, identical to that used in the USA to administer the death penalty.
Initially charged with murder in 2005, Hourmann then came to the UK looking for employment while awaiting trial.
He worked at West Suffolk Hospital, Bury St Edmunds, as an accident and emergency medic from February 2006 to August 2007.
He then moved on to Prince Philip Hospital, Llanelli, in south Wales, where he worked until July 2008.
He then moved on to work as a surgeon for Dyfed Powys Police, in Carmarthenshire, both before and after his conviction.
Hourmann, who was born in Argentina and now lives near Aberystwyth, west Wales, kept his conviction secret to the last.
He was only suspended from his police job after information was provided by an anonymous whistle-blower last month.
Police in Spain had pressed for a lengthy sentence when Hourmann stood trial for murder in Tarragona, near Barcelona.
The children of the dead pensioner had supported Hourmann throughout and had opposed charging him and putting him on trial.
He was eventually convicted of the lesser offence of manslaughter after striking a deal with the prosecution.
It meant he received only a one-year suspended jail term and was barred from holding public office for a year.
The Spanish equivalent of the GMC failed to strip him of his licence, leaving him free to work anywhere in Europe.
Dyfed Powys Police revealed that it was now investigating the circumstances of his employment with them.
“Dyfed Powys Police has a contract with an external agency for the provision of police surgeons,” said a spokesman for the force.
“We are liaising with them in relation to this matter and are investigating the circumstances surrounding this issue. The doctor has been suspended pending the results of the investigation.
“We also understand that this matter is being dealt with by the GMC and we will abide by its findings accordingly.
“Really the key thing from our perspective is that we didn’t employ him directly – we used an external service provider.”
A Carmarthenshire NHS Trust spokesman, said: “We can confirm that the doctor did work at Prince Philip Hospital for a brief period between 2007 and 2008.
“All the necessary pre-employment checks were undertaken.”
Niall Dickson, Chief Executive of the GMC, said: “Dr Hourmann has been suspended by an Interim Orders Panel.
“Our guidance could not be clearer – doctors must inform the GMC immediately of any criminal convictions they receive. If they fail to do so they may face serious consequences.
“It is important that regulators across Europe tell each other when action has been taken against a doctor – unfortunately this does not always happen as it should.
“The GMC tells other regulators when we take action against a doctor and we publish all decisions on our website, but the law in Europe does not require others to do the same.
“This is why we have been calling for changes to make it compulsory for all European regulators to share this vital information. Only when this is done will patients be fully protected.”