The law firm co-owned by Sinn Fein MP and high-profile solicitor John Finucane acted for a global drug lord when he threatened to sue several Irish newspapers.
Daniel Kinahan engaged Mr Finucane's firm, Finucane Toner, to send out legal letters after he was named as the head of one of the world's biggest drug cartels.
The company, in which Mr Finucane is one of two partners, sent out two letters in April this year.
The correspondence asked what compensation the publications intended to pay the Dublin-based narcotics kingpin for claiming he was involved in the drugs trade and had ordered multiple murders.
The letters said the allegations were "totally false and unfounded" and put Kinahan's life at risk through "reckless and irresponsible journalism".
It is understood that no libel actions were pursued in either Belfast or Dublin's High Court.
Kinahan has been named repeatedly in the Press and under parliamentary privilege as one of the biggest and most ruthless drugs bosses in Europe, if not the world.
His family's feud with the rival Hutch gang has left 20 people dead and made the front page of newspapers across the island for the past four years.
Last year a report by pan-European policing group Europol named the gang he heads as one of the main cocaine importers in Europe.
The Criminal Assets Bureau in the Republic previously said Kinahan controlled and managed the gang and has "associations that facilitate international criminal activity in Europe, Asia, the Middle East and South America".
Mr Finucane told Sunday Life: "I do not represent Mr Daniel Kinahan.
"Finucane Toner previously issued two letters on his behalf but no longer receives instructions."
Finucane Toner is a prominent law firm with an extensive criminal and civil practice.
There is no suggestion that there was anything improper in the company accepting instructions from Kinahan.
Mr Finucane was elected to Westminster for the North Belfast seat last December, having previously served a term as Lord Mayor of Belfast.
The prominent lawyer is currently representing Carl Frampton in the headline-grabbing case against his former management company Cyclone Promotions.
The Jackal is suing the company, in which Barry McGuigan was a director, over alleged unpaid purse fees.
Mr McGuigan is counter-suing, claiming an alleged breach of contract over the 33-year-old's split from the firm.
During a day of proceedings at the High Court last month, Mr McGuigan's barrister said Mr Frampton's current management firm, MTK Global, was linked to Kinahan.
Mr Frampton said during the hearing that he did not suspect the company had any ties to Kinahan but had heard stories alleging the link.
Since his election to Parliament, though he doesn't take his seat, Mr Finucane has been criticised for the amount of time he spends on legal work.Insult
The 40-year-old is one of only two directors of the firm he established with colleague Ciaran Toner in 2014.
Last month Newtownabbey Ulster Unionist councillor Robert Foster told the Irish News that seeing Mr Finucane representing clients while not taking his seat "adds insult to injury".
"The people of North Belfast face many problems and challenges. It is bad enough that they have an MP who refuses to attend Westminster and represent them by speaking up for them in the chamber of the House of Commons, but to see him representing clients in a Belfast court merely adds insult to injury," Mr Foster added.
Sinn Fein previously said Mr Finucane was "focused on his role as an MP" and had "taken a back seat in the day-to-day work" of his law firm.
According to his register of financial interests on the House of Commons website, Mr Finucane receives £4,672 a month in return for an expected monthly commitment of between 40 and 80 hours.
Until July this year he had registered the same monthly salary, but for 40 hours of work per week, rather that per month.