"As I say, it was an honest mistake and they have apologised".
"We take pairing very seriously and we recognise its value to Parliament and we will continue to guarantee a pair for MPs that are now pregnant or have a newborn baby", the prime minister added.
Conservative chief whip Julian Smith and Tory chairman Brandon Lewis have been backed by Theresa May in the face of intense pressure on them to quit in a row over claims they adopted murky tactics during a crucial Brexit vote.
"No other pairs offered on the trade bill on Tuesday were broken".
But the actions didn't have a bearing on the final result, with the Government narrowly scrapping through by 307 votes to 301 in what could have been a humiliating defeat for the Prime Minister.
Almost all of them ignored the instruction, but Conservative Party Chairman Brandon Lewis followed it and voted with the government in two knife-edge votes on Tuesday evening, one of which the government lost.
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.
"This Government is rotten to its core".
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Speaking to the Press Association during a visit to Northern Ireland on Thursday, the Prime Minister said: "An honest mistake was made here and both the Chief Whip and Brandon Lewis have apologised to Jo Swinson, the member for East Dunbartonshire, about that".
A furious Miss Swinson said she had been "assured" that pregnant MPs would be paired when necessary - and she criticised the Government for betraying her trust. MPs and political commentators have called on Smith to resign, with some insisting he should be sacked if he doesn't walk himself.
After Mr Lewis was shown to have participated in the votes, Ms Swinson accused the government of "desperate stuff". On such occasion, whips from the governing and opposition parties often make arrangements between themselves to make sure that such absences do not cause the government to win a vote that they would otherwise lose (or vice versa).
MPs voted in favour of "proxy voting" earlier this year.
Meanwhile, when asked if Mr Smith should resign, Defence Secretary Gavin Williamson, a former chief whip, said: "No, I don't".
House of Commons leader Andrea Leadsom said it was vital that new parents be able to spend time with their babies, but said there would be no inquiry as the incident would not have affected yesterday's votes.
May said on Wednesday that it had been "done in error" and was unacceptable.
So it was understandable that Jo Swinson was seething with anger when Brandon Lewis committed the ultimate Commons crime against her on Tuesday night. This means ensuring that all of their party's MPs vote - and that they understand which way the party wishes them to vote.