Rosie Duffield has accused Sir Keir Starmer of having a “problem with women” and said “the lads are in charge” in No 10.
The Canterbury MP, whose relationship with the Prime Minister has long been strained over the issue of trans rights, quit Labor over the weekend.
Asked if she thinks he has a “problem with women”, Ms Duffield told the BBC's Sunday with Laura Kuenssberg programme: “I'm afraid I do, yes.
“I mean I have experienced it myself, but most backbenchers I'm friends with are women and most of us refer to the men that surround him, the young men, as 'the lads' and it's very clear that the lads are in charge .
“They have now got their Downing Street passes, they are the same lads who were briefing against me in the papers and other prominent female MPs and I was really hoping for better but it wasn't to be.”
Ms Duffield resigned as Labor whip on Saturday with a scathing letter to Sir Keir.
She cited policies such as winter fuel payment cuts and the ongoing row about gifts and hospitality received by the Prime Minister and other senior party figures.
Pat McFadden, the Chancellor of the Duchy of Lancaster, rejected Ms Duffield's claim that “the lads” were in charge in Downing Street.
Asked whether he was one of “the lads”, he told the BBC: “I think I'm a bit too old to be a lad.
“Some of the stuff in the letter I just don't accept. I see ministers turning up to work every day and what's on their mind is how to stabilize the economy and get it growing again, how to turn around the NHS, how to get more houses built, how to improve rights at work for people, how to get more opportunity into schools.
“That's what the ministers around that Cabinet table are focused on. They believe in public service.”
Mr McFadden said he was “disappointed” but “not surprised” by her decision.
He added: “When I read Rosie's letter last night and listened to the interview there, I think you can see she has been disillusioned with the party leader, maybe the party more generally, for quite a long time.
“I don't think this is something that just developed in the last few months.”