The Chief Whip is facing mounting criticism from Tory colleagues and calls to resign after it emerged that he urged Tory MPs to break with "pairing arrangements" to avert a damaging Brexit defeat.
"Pairing" is a Parliament tradition which sees an MP from one party agree not to vote if an opposing MP is unable to attend the House of Commons.
Prime Minister Theresa May said on Wednesday that the failure to honor the pairing convention was not "good enough" but wouldn't commit to introducing rules to establish a system of voting during maternity leave.
Asked whether she endorsed her chief whip's conduct, Mrs May said: "There was an honest mistake made for which the chief whip and Brandon Lewis have both apologised to the member concerned".
Senior Labour MP Harriet Harman is leading current efforts for MPs to be given six months' maternity leave, with provisions for colleagues to vote on their behalf during their absence.
Labour has called for the chief whip and party chairman to resign if they can not fully explain what happened.
In what has been another chaotic week for Theresa May, she now finds herself under intense pressure to apologise for misleading MPs about what happened and told she must sack the two men if they do not go on their own accord.
EU Commission calls on members to step up Brexit preparations
If that deal is not sealed in time, there would be no transition period either, and thus no extra time to prepare. Those bills are critical to Britain's future working relationship with Brussels.
"This Government is rotten to its core".
Conservative MP Heidi Allen said: "No matter how tough the going gets, principle, integrity and standards matter. If true, Julian Smith must resign, or be sacked".
However, the Times reported that Mr Lewis and two other Tory MPs were ordered by Mr Smith to vote despite being paired. "If we can not behave with honour we are nothing". A report by the cross-party Commons procedure committee has already set out how one could be set up, including proxy voting for parliamentarians who are absent after giving birth or through illness, but no action has been taken yet.
Ms Swinson appeared to mock the controversy, and said: "This reflects pretty badly on those peddling the "honest mistake" nonsense".
A Conservative spokesman stopped short of denying the reports.
Downing Street insisted Mrs May still had total confidence in Mr Smith.
There were cheers from MPs on the government benches after it was defeated, as Theresa May's administration swerved a second defeat on the Trade Bill.
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