Los Angeles (AFP) - Conor McGregor hit back at former sparring partner Paulie Malignaggi, saying the retired two-time champion was looking for an excuse to leave training camp.

Mixed martial arts star McGregor is preparing to face Floyd Mayweather in a Las Vegas boxing ring on August 26 in what could be one of the richest fights in history.

McGregor has never boxed professionally so he brought in several sparring partners to help him get ready to take on the undefeated Mayweather, who is considered one of the greatest of all-time.

"He was looking for an exit," McGregor said of Malignaggi at his training facility in Las Vegas. "The spars were not good for him. He took a lot of head trauma. Straight after it we were worried. The sparring partners were actually telling us when he was dropped back at the home that he was stumbling out of the car after the 12-rounder.

"He was flattened against the ropes; multiple times the ropes kept him on his feet. We were worried about him. We thought the worst was about to happen. He was certainly badly concussed. And then he just got out of there and went running."

Malignaggi scoffed at McGregor's comments, taking to social media to defend his earlier comments that he walked out because McGregor was using him as a pawn to try and give credibility to a "house of cards" training camp.

"Let them have their 35 seconds. They know what happened last week. It's a house of cards. (Two) weeks left," Malignaggi tweeted on Saturday.

The 36-year-old said he agreed to become a sparring partner because he thought McGregor wanted to train with a high-level boxer ahead of the Mayweather clash.

Malignaggi, who retired from boxing five months ago, walked out earlier this month after he said McGregor disrespected him in the ring and the dressing room and housed him and the other sparring partners in accommodations resembling a "crackhouse".