Senior Labour MP Christina Rees has had the party whip suspended following reported allegations of bullying.
The Neath MP – who was shadow Welsh secretary under Jeremy Corbyn – will now sit in the House of Commons as an independent.
She is understood to have had her party membership administratively suspended while the complaint is investigated.
The move followed complaints by staff working for her in her South Wales constituency, according to The Guardian.
Local party members told the newspaper that several members of staff had made claims of bullying and provided evidence to Labour HQ.
In a statement to the newspaper, Ms Rees said: “There has been a complaint made against me to the Labour Party, which is under investigation and I am therefore under an administrative suspension until the process is concluded.”
The MP added: “I’m not aware of the details of the complaint but I am fully co-operating with the investigation.”
A Labour spokeswoman said: “We do not comment on individual complaints.”
The latest YouGov poll showed Labour enjoying a 28-point over the Tories, as Liz Truss’s government flounders in the wake of the disastrous mini-Budget.
Sir Keir Starmer warned Labour staff to be ready for the collapse of the government and a general election “at any time”, as he revealed a shake-up of his top team on Wednesday.
It comes as Len McCluskey branded Labour leader Sir Keir a “liar” and claimed “seeking to kill off the socialist heart of the Labour party”.
Mr McClusky also told GB News that Mr Corbyn – suspended by the party over his response to a major antisemitism report – should stand as an independent MP.
“Without a shadow of a doubt. He was thrust into the centre of leadership, but in essence, it’s his constituency,” he said.
The left-wing firebrand added: “He’s lovely, he walks round the streets, he knows people. So, yeah, I think he absolutely should run again.”
Labour MP Sam Tarry demanded to see details of the vote that led to him being deselected in his constituency.
The left-winger, sacked as a shadow transport minister after giving interviews from a picket line, failed to see off the challenge from the leader of Redbridge council, Jas Athwal.
“I am extremely concerned about the result, which does not reflect the feeling my campaigners met on the ground talking day in day out to members,” he said after Monday’s vote held after local branches opted to trigger reselection proceedings.
