A sitting Labour MP has filed a complaint against Geraint Davies after he was suspended over allegations of inappropriate behaviour, Sky News understands.
It means Labour has now received two formal complaints after a woman also contacted the party yesterday.
Mr Davies has been "administratively suspended" from the party pending an investigation into reports of "incredibly serious allegations of completely unacceptable behaviour".
He was first elected as an MP in 1997, but was out of parliament between 2005 and 2010. He now represents Swansea West.
Allegations regarding the 63-year-old politician came to light in a report by Politico published on Thursday.
He has denied the claims and told the website: "I don't recognise the allegations suggested and do not know who has made them.
"If I have inadvertently caused offence to anyone, then I am naturally sorry as it is important that we share an environment of mutual and equal respect for all."
Labour suspended the MP from the party and urged those who had been impacted by his behaviour to make a formal complaint so they could carry out an investigation.
In a separate development, a Labour MP has claimed one of her colleagues who was reported to the police for alleged sexual assault is still on the opposition frontbench because the party "has chosen not to act".
Charlotte Nichols, who represents Warrington North, did not name the individual but said she had challenged Labour officials about the claims and "their response has been utterly inadequate".
Speaking to BBC Radio Four in the wake of the Davies scandal, Ms Nichols also revealed that within her first week of being elected to parliament in 2019 she was given a "verbal list" of around 30 MPs she should "steer clear of" due to their inappropriate behaviour.
She said there is a "deeply rotten culture" within Westminster when it comes to sexual harassment and "none of the systems we have in place are effective".
Giving an example, Ms Nichols said: "One of our colleagues who has been reported to police for assault who still has the party whip is still on the frontbench, but the party has chosen not to act."
Ms Nichols also said: "I am aware of cases where people have been sexually assaulted, they have gone to the ICGS (parliament's independent complaints and grievance scheme) and have been told that they can't act because it didn't happen on the parliamentary estate."
The backbencher said that "fundamentally there's an underlying rotten culture in Westminster and it's baked into every level of how the institution operates".
"It's incumbent on all of us to stop talking about bad apples and talk about what's causing that rot."
Last month, it was reported by Tortoise Media that a female Labour MP claimed she had been sexually assaulted by a male shadow minister, allegedly after a summer party in London in July 2021.
The BBC later reported that a complaint was made to the Met Police but they would not be investigating the matter "at the victim's request".
At the time, Labour said it had not been contacted by the police or received a formal complaint.
In the wake of the Davies claims, a party spokesperson said: "We would strongly urge anyone with a complaint to come forward so that allegations can be swiftly and fully investigated and action taken.
"The party has ensured that there is a wide range of support available to complainants, to provide confidence and confidential guidance throughout the disciplinary process."