The New York times reported Tuesday that the 30-year-old Dubliner is under investigation relating to a sexual assault accusation from a young woman, according to sources familiar with the case. 

McGregor has not been charged with the crime, but was arrested, questioned by police in connection to the accusations, before being then released without charge in January this year. The allegations have yet to be proven.

Nicknamed 'The Notorious', McGregor announced he was calling time on his fighting career, also on Tuesday, taking to Twitter to inform his followers of the decision.

"Hey guys quick announcement, I’ve decided to retire from the sport formally known as “Mixed Martial Art” today," he wrote.

"I wish all my old colleagues well going forward in competition. I now join my former partners on this venture, already in retirement. Proper Pina Coladas on me fellas!"