Conor McGregor, the Ultimate Fighting Championship's (UFC) biggest star and one of the world's highest-paid athletes, is under investigation in his native Ireland on an accusation of sexual assault, according to New York Times.

The report came a day after McGregor's announcement on Twitter that he had decided to retire from his career inside the octagon. 

"Hey guys quick announcement, I've decided to retire from the sport formally known as 'Mixed Martial Art' today," McGregor wrote. "I wish all my old colleagues well going forward in competition."

The New York Times noted that McGregor has not been charged with a crime and the fact that an investigation is continuing "does not imply that McGregor is guilty of a crime."

It is also noted that his accusation and retirement are not related.

It was the second time that McGregor had announced on Twitter that he was leaving the sport. 

A month after losing to Nate Diaz in March 2016 he said he had decided to “retire young”, a move that was widely seen as a negotiating gambit, as it only lasted a matter of days and he subsequently signed an agreement to fight Diaz later in the same year, winning a five-round war of attrition on points.

The 30-year-old has not fought since being well beaten by rival Khabib Nurmagomedov last October.

McGregor was later handed a six-month suspension for his role in a post-fight melee after the Nurmagomedov bout, a ban that would have left him eligible to fight again early next month.

He was arrested and charged with criminal mischief and strong-armed robbery in Miami earlier this month as he had slapped the phone out of a fan's hand and then stomped on it.

(With input from AFP and Reuters)