THERESA May has apologised after one of her MPs welched on a deal not to vote while Lib Dem Jo Swinson is on maternity leave.

Tory chairman Brandon Lewis was “paired” with the East Dunbartonshire MP for Tuesday night’s crunch Brexit. This meant neither should have voted and they would have effectively cancelled each other out.

The archaic procedure of pairing is well established in the House of Commons, but on Tuesday night, Lewis abandoned it.

He went through the lobbies with the government, against an amendment to the EU trade bill that would have kept the UK in a customs union.

The Tories won by just six votes.

Swinson, who gave birth to her son Gabriel at the end of June, was furious and accused the Tories of cheating.

During Prime Minister’s Questions, May told MPs: “The breaking of the pair was done in error. It wasn’t good enough and will not be repeated.”

The PM said the chief whip and the Tory party chairman had “apologised directly”.

She added: “We take pairing very seriously and we recognise its value to Parliament and we will continue to guarantee a pair for MPs that are currently pregnant or have a newborn baby.”

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

Lib Dem chief whip Alistair Carmichael, secured an urgent question in the Commons.

He told MPs it was “symptomatic of a wider problem”.

He said: “It is using a 19th century practice to provide for cover under 21st century employment law and that is no longer good enough.”

Carmichael went on to tell MPs that the situation had improved since he was first elected in 2001, when his son was 10-weeks old, but more needed to be done.

He said: “When I was first elected children were not to be seen and certainly not to be heard within the House.

“I remember I once had to change my younger son’s nappy in the member’s cloak room where obviously he shouldn’t have been because he wasn’t a member, on a copy of the Daily Record because there was no changing mats.

“We have come a long way, but anybody who thought that we had done it all and there was no more left to be done was sadly disabused last night.”

Andrea Leadsom, replying on behalf of the Government said it “was not good enough” and again offered an apology on behalf of the Government.

She added: “I’m assured by the chief whip that the breaking of the pair yesterday was done entirely in error and will not be repeated.”

The SNP’s Pete Wishart said the Tory failure to keep their commitment on Tuesday was why his party has chosen not to take part in pairing.

“Pairing relies on trust and I am sorry, but we are absolutely right not to place our trust in Government Members,” he told MPs.

“We have to change the voting arrangements of this House. We see that every day in the absurd waste of time of a headcount in cramped voting Lobbies, but to be disenfranchised for having a baby in 2018 demonstrates just how out of touch this archaic place is and how these arrangements should embarrass and shame this House.”

The Speaker said a decision on proxy voting must be made soon.