Conservative Chief Whip Julian Smith must resign if he is privately blocking proposals to allow MPs on maternity leave to vote by proxy, Liberal Democrat deputy leader Jo Swinson has said.

The Times reported that Mr Smith was trying to “obstruct” a change in the voting system as he attempts to win support in the Commons for Theresa May’s Brexit deal.

Last summer he ordered Tory Party chairman Brandon Lewis to break a pairing agreement with Ms Swinson while she was on maternity leave during a crucial Brexit vote.

Ms Swinson said: “Julian Smith had to apologise for cheating my constituents out of their voice, by asking an MP to break our pair while I nursed my two-week-old baby.

“If he is blocking proxy voting in private, it makes an absolute mockery of his public apology and he should resign.”

But the Prime Minister’s official spokesman said proxy voting was “still something we are looking at”.

He told reporters on Monday morning: “The Prime Minister discussed this in the House and said it was under active consideration, but it is important that we make sure any changes we make are done in the right way.

“That remains the case. It is still something we are looking at.”

Last week Speaker John Bercow said there had been a “lamentable failure of leadership” to introduce changes to allow a colleague to register a vote on behalf of an MP absent on maternity or paternity leave.

His comments came after a heavily pregnant Labour MP, Tulip Siddiq, postponed a Caesarean section in order to vote on the Prime Minister’s Withdrawal Agreement last week.

Pairing agreements are used when MPs cannot make it to the Commons for votes for medical and other reasons.

It means someone from the other side also does not vote, to ensure there is no unfair advantage caused by their absence.

Mr Smith’s office did not respond to a request for comment.