The 61-year-old Spandau Ballet bassist and former EastEnders star has admitted that lead singer Hadley took "the brunt" of jokes in the band and said in his new book Ticket to the World: My 80s Story it “tipped over into bullying”.
Kemp told The Guardian: “If you imagine five boys in the playground, there’s always going to be one guy that’s going to take the brunt of the jokes.
“Now, I’m not saying that Tony was bullied, as such... but he took all the banter.
"I can look back at it now with space. And I don’t like the way that we used to group up on Tony, and I feel guilty about that… If it was me, I think it would have been too much for me."
The True singer - whose brother Gary Kemp was also in the band, playing guitar and writing the songs - admitted the behaviour was mainly driven by jealousy of Hadley's singing voice.
He said: “It was mainly singer envy. If you asked Tony about it, he might not even say he recognised it. But I recognise it. And there was part of me that recognised it at the time."
Hadley, 62, did not rejoin the Gold group when they reformed in 2017.
He said earlier this year: "I have nothing to do with Spandau at all now. I wish they would be honest and someone would ask them: ‘Why did your lead singer leave the band?’
"It’s not just, as they said in the past, ‘He didn’t want to do it any more’. That’s a lie."
Spandau Ballet formed in London in 1979 and rose to fame in the 1980s as one of the most successful examples of the New Romantic era.
They had huge hits with tracks including Gold and True, releasing six studio albums before they split in 1990.
In 1999 Hadley, drummer John Keeble and sax player Steve Norman failed in an attempt to sue Gary Kemo for a share of royalties to the songs.
In 2009, they reformed in the original line-up and embarked on a tour as well as releasing their seventh album.
In 2017 they reformed with Ross William Wild replacing Hadley as lead singer.
The band said in a statement: "Much to our frustration, Tony had made it clear in September 2016 that he didn’t want to work with the band any more.
"This has not changed and 2015 was the last time we were able to perform or work with him. So we have now made the decision to move on as a band."
The group has been inactive since 2019, when Wild announced that he too was stepping away from the band.
Watch: Martin Kemp on Spandau Ballet/Duran Duran feud