Michael Avenatti has offered to fight Donald Trump Jr for charity in the latest barbed exchange between the attorney and America's first family.

Mr Avenatti, who is representing adult actress Stormy Daniels in her lawsuit against president Donald Trump, revealed his plans to organise the bout during a meeting with a reporter.

The lawyer offered Mr Trump Jr the opportunity to take part in a three-round mixed martial arts (MMA) fight, the proceeds of which would be donated to charity.

“This would be for two great causes. I’m in,” Mr Avenatti said on Twitter, adding he was waiting for Mr Trump Jr to confirm the event.

He said his proceeds would be split between hurricane-hit Puerto Rico, where the president claimed the official death toll was being exaggerated by Democrats to make him “look bad”, and the Rape, Abuse and Incest National Network (RAINN).

The attorney’s proposal came following an earlier clash with Mr Trump Jr, a figure Mr Avenatti has often argued with on social media.

The president’s son had tweeted news of disgraced Democratic congressman Anthony Weiner’s potential release from prison, where he is serving a 21-month sentence for sending obscene texts to a minor.

“Who else is excited about Avanetti [sic]/Weiner 2020?!?” Mr Trump Jr wrote, indicating he could imagine the two Democrats running against each other for their party's nomination for the 2020 US presidential election.

“It’s gonna be Carlos Dangerous,” he added, referencing the “Carlos Danger” pseudonym Wiener used when sending explicit pictures of himself to a 22-year-old woman during a separate scandal in 2013.

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But Mr Avenatti responded claiming Mr Trump Jr could himself face a spell behind bars.

“If I were you, the last thing I would be doing is referencing other people getting out of federal prison. Because after you are indicted, you will likely be passing them on your way in,” he wrote.

“BTW, they don’t have silver spoons or gold toilets in the joint. Buckle up Buttercup.”

Mr Avenatti, a vociferous critic of the Trump administration, has previously indicated his interest in a run to oppose the president in 2020, saying the Democratic candidate needs to be a “fighter”.

Former White House strategist Steve Bannon has also played up Mr Avenatti’s credentials, claiming he could be a serious contender for the nomination.

“[Avenatti]'s got a fearlessness,” Mr Bannon told political talk show host Bill Maher last month.