Tory chairman Brandon Lewis has been accused of breaking an agreement not to vote on crucial Brexit legislation.
The Conservative MP was “paired” with Liberal Democrat deputy leader Jo Swinson, who recently gave birth, meaning neither would take part in the Commons votes.
But the senior Conservative did vote and helped Theresa May narrowly avoid a humiliating Commons defeat on Brexit trade plans.
Mr Lewis apologised and claimed it was an “honest mistake” in “fast-moving circumstances”.
The pairing system operates by making sure that if an MP on one side of an argument is unable to vote, an opponent also sits out the division to even it out.
But a furious Ms Swinson revealed that Mr Lewis had reneged on the deal, asking Theresa May: “Just how low will your Government stoop?”
She said it was “desperate stuff” and dismissed the Conservative response that Mr Lewis had voted by mistake.
Accusing the Tories of “cheating”, she said it was a “calculated, deliberate breaking of trust” by the Government whips to “win at all costs”.
She added that the Government’s response to discrimination against pregnant women and new mums had been “shockingly poor”, so “perhaps it should be no surprise they treat MPs on [maternity] leave like this”.
Mr Lewis had missed earlier votes, but took part in the two crunch divisions on the Trade Bill.
Shadow women and equalities minister Dawn Butler said it was an “absolutely appalling move” by the Tory chairman, and called on Mr Lewis to personally apologise.
The Government avoided defeat by just six votes over an amendment which could have required the UK to join a customs union after Brexit if a trade deal was not in place by January.
Ms Swinson gave birth to her son Gabriel on June 29.
Mr Lewis told her: “I’m sorry Jo. I think it was an honest mistake made by the whips in fast-moving circumstances.”
Chief Whip Julian Smith also apologised to Ms Swinson, saying on Twitter he was “v sorry” and Mr Lewis was “asked to vote in error”.