According to Grazia magazine, Russ was left "seething with jealousy" after seeing snaps of his ex-wife kissingFlorence + The Machine guitarist Robert Ackroyd, and reportedly sent Perry an email suggesting the divorce was a 'mistake'.
Brand filed for divorce from the Firework hitmaker in December last year, citing "irreconcilable differences" as the reason for the separation, but Grazia reports he confessed to "giving up" on the marriage too quickly in an emotional email to Katy.
A source told the publication: "Word has it Russell was 'seething with jealousy' when he saw pictures of her with Rob. He apparently feels he gave up on their relationship too easily.
"It's fair to say that Russell only wishes Katy happiness but perhaps he should have realised just how special his wife was when he was married to her rather than five months later."
At a stand-up gig last week, Russell allegedly launched into a rant at an audience member who reportedly shouted out his ex-wife's name.