Allies of Yvette Cooper and Liz Kendall have accused leadership rival Andy Burnham of mounting a macho, "male-dominated" campaign.
The race took a bitter turn when Lord Falconer, a high-profile Burnham supporter, said neither Kendall nor Cooper had what it takes to be leader.
"Liz Kendall and Yvette Cooper are both very talented politicians," Falconer wrote. "Both have a big future in our party. But neither Yvette nor Liz can steer the Labour party through the challenging few years ahead of us when we need a leader who can reach out to all wings of our party and provide unity. As a result, both Liz and Yvette are unlikely to beat Jeremy [Corbyn]."
Senior Labour MP Barry Sheerman, who is backing Kendall, told BuzzFeed News: "I've know Charlie Falconer a long time, I'm on good terms with him, but he seems to have just lost the plot.
"He's a man who's never run for election in his life and seems to now be pontificating on how strong our two women candidates can be. Now, I know both Yvette and Liz, and they've got a core of steel – stronger than either of the two male candidates."
Sheerman branded Burnham's leadership drive "a very macho campaign, a very male campaign". He added: "When he announced at the beginning he was the 'change candidate' and then appointed hatchet men to run his campaign, I think we took that with a pinch of salt."
A female Labour MP added: "I think this says a lot about the male-dominated machismo of the Burnham campaign. He just doesn't see women. It will be a disaster if Labour selects two men at the top of the party."
Cooper's camp weren't much impressed either, claiming that the article raised questions about Burnham's attitude towards women.
A source close to Cooper's campaign said Falconer's piece would have been written with the full backing of Burnham's team.
"The question is, does Andy Burnham believe women are up to the job?" the source told BuzzFeed News. "I think we would be very happy putting Yvette Cooper's record in government and as shadow foreign secretary and shadow home secretary against Andy's or any man."
Falconer's piece also prompted a stinging response from Kendall, who said it was "depressing to see a senior man in the party dismiss the contribution of women so easily".
She said: "For Charlie to say that women somehow aren't tough enough to lead the Labour party is a gross insult, and as for standing up to Jeremy Corbyn, I'm the only candidate who has been saying he would be a disaster for our party and that I wouldn't serve in his shadow cabinet, unlike the candidate Charlie is supporting."
Another female frontbencher added: "Falconer's comments are totally unacceptable and give the message of 'leave it to the boys'. Neither Liz or Yvette are weak women or not tough enough. It's quite ridiculous."
Burnham's team insisted it was "nonsense" and "offensive" to suggest that Falconer's piece was sexist.
A source in Burnham's camp said: "It was a disgraceful and utterly misleading headline from The Times. It was absolutely not what Falconer was arguing, it had nothing to do with gender but was simply about supporting the best candidate.
"Andy has lots of support from female MPs and has made it very clear that he wants 50-50 representation on the shadow cabinet and a female first secretary."
Emily Ashton is a senior political correspondent for BuzzFeed News and is based in London.
Contact Emily Ashton at email@example.com.
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