Karl Turner is alleged to have slapped a woman, who had recovered from a double mastectomy, on her bottom in his constituency office in 2015.
The MP for Kingston upon Hull East is also alleged to have told the woman she “shouldn’t have got rid of her real tits because they were great”.
The shadow transport minister has denied behaving inappropriately in response to the claims.
Anonymous witnesses told the Financial Times about the exact nature of Mr Turner’s comments.
The claims come with six Labour MPs, including Mr Turner, facing allegations of bullying or sexual harassment.
Labour insisted it took all complaints about inappropriate behaviour “extremely seriously” and appealed for anyone with a complaint to contact the party to allow allegations to be fully investigated.
After the alleged bottom-slapping incident, Mr Turner reportedly said something like “I couldn’t help myself” when he was challenged.
He allegedly followed up his comments about the woman’s mastectomy by adding: “The ones you’ve got now are nice enough.”
Mr Turner said: “I am aware of reports in the media about my alleged inappropriate behaviour.
“I strongly reject any suggestion that I behaved inappropriately or in a misogynistic manner.”
A Labour spokesman said: “The party takes all complaints of sexual harassment, abuse and discrimination extremely seriously.
“We ask that anyone with a complaint comes forward so that allegations can be fully investigated, and any appropriate disciplinary action taken in line with the party’s rule book and procedures.”
Labour has refused to investigate, saying a complaint from a victim was required.
Debbie Abrahams was suspended on Sunday as shadow pensions secretary over separate accusations of bullying staff.