Kubrat Pulev is expected to face Anthony Joshua in a title fight on December 12.
But he has been accused of racism after referring to Joshua's skin colour.
When asked about his verdict on Joshua, Pulev sparked fury with his answer.
The pair were meant to fight in June but it was postponed due to coronavirus.
Kubrat Pulev sparked accusations of racism ahead of his world heavyweight title fight with Anthony Joshua by claiming 'his tan is better than mine'.
The 39-year-old Bulgarian will battle it out it with Joshua for his WBA, IBF and WBO titles, with the fight set to take place on December 12 after it was initially postponed due to the Covid-19 outbreak.
And ahead of the pair's upcoming title fight, Pulev was asked about his verdict on Joshua - in which he made a controversial comment about his opponent's skin colour.
'His strengths? I'd say he has a very powerful punch undeniably,' said Pulev during a press conference.
'What are his other strengths? His tan is better than mine.'
Joshua - who is of Nigerian and Irish ancestry - has been a big advocate of the Black Lives Matter movement in the fight against racism in society.
Joshua even delivered a powerful speech as he showed his support for the movement when he joined a protest march in his home town of Watford. The world champion joined hundreds of others on the peaceful protest.
And following Pulev's comments regarding Joshua, many boxing fans let their feelings be known on social media.
After hearing what Pulev said, one fan wrote: 'That's a shameful 1970s comment. Awful.'
'Kubrat Pulev sadly proving why #BlackLivesMatter is needed. Idiot!', another boxing follower said on Twitter.
And the condemnation of the Bulgarian's comments kept on coming on social media on Thursday.
Another fan said: 'I hope Anthony Joshua smashes the living daylight out of Pulev after that "tan" comment.'
Someone else added: 'I think Joshua will take that Pulev comment and ram it down his throat. Expect him to want to hurt him.'
The Bulgarian's comments come after the country's football team were forced to play two matches behind closed doors for their fans' racist abuse towards England players in a Euro 2020 qualifier last year.
England's 6-0 victory over Bulgaria was stopped twice and was close to being abandoned, but Gareth Southgate's side chose to play on in Sofia.
Bulgaria were subsequently fined a mere £65,000 by UEFA.
The Bulgaria supporters' horrific actions included Nazi salutes and monkey chants towards England's black players.
President of the Bulgaria Football Union, Borislav Mihaylov, and Bulgaria boss Krasimir Balakov resigned as part of the fall-out from the game.
Twelve arrests were made, with four supporters handed fines and two-year bans.
Meanwhile, it is not the first time that Pulev has landed himself in hot water.
Last year, the Bulgarian kissed a female reporter on the lips following his knockout victory over Bogdan Dinu.
After he was heavily criticised online, the boxer then insisted the pair were actually 'friends'.
'You may have seen a clip of me kissing a female reporter following an interview after I won my fight Saturday night,' Pulev said.
'The reporter, Jenny, is actually a friend of mine, and after the interview, I was so elated, I gave her a kiss.
'Later that night, she joined me and my other friends at my post-fight celebration. On the video, after our kiss, we both laughed about it and thanked each other. There really is nothing more to this.'
But the reporter, Jennifer Ravalo, gave a different version of events, claiming she was left 'immediately shocked and embarrassed.'
Pulev had been due to take on Joshua at the Tottenham Hotspur Stadium in June but that was rescheduled due to the pandemic but he has now confirmed it will go ahead on December 12 in London.
'No turning back, no procrastination! Kubrat Pulev vs Anthony Joshua! The date is December 12th and the place is London!,' the Bulgarian, known as 'The Cobra', said on his Facebook page recently.
Pulev became the mandatory challenger to Joshua's IBF crown after beating Hughie Fury by unanimous decision back in October 2018.