London(CNN) A British lawmaker on maternity leave has accused Theresa May's government of "cheating" after it broke an agreement that cancels out the votes of politicians who are unavoidably absent from parliament.
Jo Swinson, the deputy leader of the Liberal Democrat party, believed she did not need to be present for a key Brexit vote on Tuesday because a member of May's Conservative party had agreed not to cast an opposing ballot.
But the Conservative MP in question, party chairman Brandon Lewis, voted in support of the government, contributing to a narrow, six-vote win for May's beleaguered administration.
Swinson said Lewis had broken a longstanding arrangement that allows British lawmakers who are absent from parliament to be "paired" with opposing MPs who agree not to vote, thereby balancing out the tally.
Wrtining on Twitter Swinson described the move as "desperate stuff" and said the government "broke an agreement" that benefited pregnant women and new mothers in parliament.
"Just how low will your govt stoop @theresa_may?" she wrote.
"This is calculated, deliberate breaking of trust by govt whips... to win at all costs," she added. "There's a word for it -- cheating."
Lewis apologized on Twitter, describing the incident as "an honest mistake" made by party administrators "in fast-moving circumstances."
"I know how important the pair is to everyone, especially new parents, and I apologise," he wrote.
David Lidington, a senior meber of May's cabinet, also apologized to Swinson Wednesday morning, telling BBC Radio 4's Today program that it was a "genuine mess-up" that "clearly should not have happened."
Swinson rejected Lewis' explanation, arguing that it was "not credible" that the MP "forgot" he was paired for the day's two key votes, having already abstained in several others earlier Tuesday.
The Scottish MP, who announced the birth of her son Gabriel on July 1, used the incident to highlight the problem of discrimination faced by pregnant women and new mothers in the UK.
She referred to research published in May by the Equality and Human Rights Commission, showing that more than 10% of mothers interviewed said they had been dismissed or treated so poorly they felt they had to leave their jobs.
"Despite claims to want to fight 'burning injustices,' govt response to this problem has been shockingly poor, so perhaps it should be no surprise they treat MPs on mat leave like this," she wrote.
Swinson also called for urgent action to introduce a system of proxy voting, which would allow MPs to nominate colleagues to place votes on their behalf.
"Enough of this," she wrote. "Instead of closing Parliament early, let the Commons decide on proxy voting next week -- with binding motions -- and put an end to this charade."
MPs approved plans for proxy voting in February, but these have not yet become law.
Lawmakers from across the political spectrum also took to Twitter to express their outrage at Lewis' actions.
Labour MP and Shadow Foreign Secretary Emily Thornberry called the episode "disgraceful."
"When their Chair @BrandonLewis double-crosses a pregnant MP like this, the Tories show just how ruthless & desperate they are to hold onto power," she wrote.
Conservative lawmaker Sarah Wollaston called for someone to take responsibility "for failing to honour the pairing agreement for @joswinson who is on maternity leave."
"More than just an extension of the other heavy handed tactics on display, it disrespects women and why maternity leave matters," she wrote.