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Liberal Democrats today demanded urgent reforms to a long-standing Commons convention which sees opposing MPs paired up to miss votes.

The Conservatives have been accused of “cheating” after party chairman Brandon Lewis voted in two crunch Brexit decisions despite being paired with Lib Dem MP Jo Swinson, who is away on maternity leave. His interventions helped Theresa May narrowly avoid a damaging defeat on Brexit trade plans, finally by just six votes.

Chief Whip Julian Smith apologised and said Mr Lewis has been “asked to vote in error”. Mr Lewis said it was an “honest mistake” made by the whips in “fast-moving circumstances”.

But Ms Swinson said it was “neither honest, nor a mistake” and the Government’s response was “not credible”.

Accusing the Tories of “cheating”, she said it was a “calculated, deliberate breaking of trust” to “win at all costs”.

The former coalition minister gave birth to son Gabriel 20 days ago and is at home in her Scottish constituency looking after him and her eldest son, Andrew.

Mr Lewis had missed less contentious votes earlier yesterday but took part in the two crunch divisions on the Trade Bill. Under the pairing system if an MP on one side of an argument is unable to vote, an opponent also sits out the Commons division to even it out.

A Lib Dem party source said: “We shouldn’t be relying on a gentlemen’s agreement to make sure that women who have babies have proper maternity leave. A promise isn’t worth anything.

“Trying to squeeze 21st legislation into a 19th century gentlemen’s agreement just doesn’t work any more. They should be able to nominate a proxy. What we need now is proper reforms. We need an apology with action.”

Conservative MP Sarah Wollaston tweeted that the incident had been “more than just an extension of the other heavy-handed tactics on display” during the knife-edge vote. She said: “It disrespects women and why maternity leave matters.” Shadow foreign secretary Emily Thornberry tweeted: “When their Chair @BrandonLewis double-crosses a pregnant MP like this, the Tories show just how ruthless & desperate they are to hold onto power.”

Labour MP Cat Smith, who has just given birth, and pregnant Labour MP Laura Pidcock had pairing arrangements which worked. The Government is understood to be considering proxy voting.