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R. Kelly’s lawyer believes Michael Avenatti’s arrest on federal extortion charges could torpedo the singer’s sex abuse case, according to a report Tuesday.

Steve Greenberg is calling into question the “chain of custody” of the alleged sex tape believed to be the smoking gun in the case against Kelly, TMZ reported.

He said there’s a possibility that the tape — which allegedly shows the R&B star engaged in a sex act with a minor — could’ve been altered before it landed in prosecutors’ hands.

Avenatti, who represents several of Kelly’s alleged victims and their family members, got the damning evidence from a person who’s extorted Kelly before, sources told the gossip site.

The man recently confessed to Chicago prosecutors that the “I Believe I Can Fly” singer’s team forked over about $1 million to prevent the tape from going public in the past — raising the question of how the recording wound up in Avenatti’s possession.

Avenatti said he turned over two sex tapes to prosecutors but Greenberg told TMZ he’s only aware of one.

Kelly is facing 10 counts of aggravated criminal sexual abuse of four victims, including three minors.

Avenatti, meanwhile, was arrested Monday on charges that he tried to extort more than $20 million from Nike by threatening “to release damaging information” about the sneaker giant, according to court papers.