The Tory failure to stick to a voting pact with a senior opposition MP on maternity leave was not “good enough”, Theresa May has said.

Conservative chairman Brandon Lewis was “paired” with Liberal Democrat deputy leader Jo Swinson, meaning neither would take part in the Commons votes.

But the Tory MP then took part in two crunch Commons votes on Tuesday night and helped Theresa May narrowly avoid a damaging defeat on Brexit trade plans.

During Prime Minister’s questions, Mrs 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.”

Labour said it was “hard to credit” the account of the incident given by Mr Lewis and chief whip Julian Smith and called for fuller details.

A spokesman for Labour leader Jeremy Corbyn said: “We want a full explanation of what took place, given that he only seems to have been unaware about the pairing for the two closest votes.

“It is hard to credit the explanation that has been given.”

Mrs May said the Government was “looking very careful” at proposals for proxy voting.

“We are looking at the interests of individuals but also the interests of the whole House,” she added.

Mr 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”.

Mr Lewis had missed earlier votes, but took part in the two crunch divisions on the Trade Bill.

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.

A furious Ms Swinson revealed that Mr Lewis had voted, asking Mrs May: “Just how low will your Government stoop?”

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”.

Shadow women and equalities minister Dawn Butler said it was an “absolutely appalling move”.

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.”

Mr Smith also apologised for the mistake and said he had spoken to his Lib Dem counterpart Alistair Carmichael about it.

Mr Carmichael, who secured an urgent question on the matter, later told ministers that events were “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.”

The Lib Dem chief whip 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 were 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.”

Ms Leadsom responded saying the situation that transpired “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.”

Speaker John Bercow said MPs should “decide the issue” when it is debated, adding: “I think there is a concern about potentially an endless debate.”

Shadow Commons leader Valerie Vaz urged the Government to come to an agreement on Wednesday with other parties to “nod through” those MPs on baby leave for every vote.

She added: “We could ensure those voting by this means are denoted with a ‘P’ or, to make it really up to date, and I hope Hansard has this, a baby emoji, therefore giving full transparency to the public.”

Tory MP Heidi Allen (South Cambridgeshire) urged the Commons to “modernise” before saying: “Dragging in sick and heavily pregnant members does not send a good message to the public – it’s not good enough for us to be OK in here, we have to be better than OK in everything we do.”