A Liberal MP chairing a controversial inquiry into Labor’s proposed changes to franking credits is being urged to resign over “highly unethical” behaviour.

Tim Wilson has been under fire for including a Liberal petition against the policy on a website promoting the committee’s work and allowing his colleagues to hand out party membership forms at public hearings.

It has now been revealed Mr Wilson holds shares in a fund management firm leading the charge against Labor’s changes and is related to its chairman.

Labor’s Matt Thistlethwaite told The Sydney Morning Herald that Mr Wilson must resign, arguing he has a clear conflict of interest and abused parliamentary processes.

“Not only did he create a blatantly partisan website but it now appears he has an interest in a company that is leading the charge against Labor policies,” he said.

“That is highly unethical and it taints the inquiry.”

However, Mr Wilson is unrepentant.

“What’s unethical is you and Bill Shorten trying to rig the law to smash retirees and steal their overpaid tax unless they join a Labor-linked union super fund stacked with your mates,” he tweeted to Mr Thistlethwaite.

“At every point Labor has been doing the bidding of their mates against the Australian people.”

The economics committee is holding public hearings across the country on Labor’s plan to remove cash payments for excess franking credits.

Unusually, there are no formal witnesses scheduled for most of the hearings. Instead, members of the public are given three minutes each to vent their feelings.

Hundreds of self-funded retirees have shared concerns about what the proposed changes might mean for their incomes.