Sir Vince Cable missed Monday's knife-edge vote on Brexit, as he was at a confidential political meeting.

Almost a year since the Richmond MP became the leader of the Liberal Democrats, he has come under scrutiny after missing on the customs vote.

He missed the vote along with predecessor Tim Farron. They were both allowed to skip it by chief whip, Alistair Carmichael.

The whip says he ‘messed up’ by allowing the pair to skip the vote and said he expected the vote to be “lost by hundreds.”

Sir Vince said: “With hindsight, I should have been there. I had an important meeting away from the Parliamentary estate and it is often difficult to reconcile diaries in this chaotic political environment.

“The votes wouldn’t have been crucial because of the block of Labour that voted with the government or abstained. But we are redoubling our efforts as the only mainstream party fighting for an exit from Brexit.”

In a statement tweeted by Sir Vince, Mr Carmichael said: "the governments winning margin should have just been one." The winning margin was three with the pair's absence.

Mr Carmichael added: “By the time it became apparent that the vote was going to be close – it was too late to get two of our MPs back in time to vote, I am taking responsibility and redoubling my efforts to stop Brexit.”

A Liberal Democrat spokesperson said: “Sir Vince Cable was at a confidential political meeting outside the parliamentary estate."

Mr Farron was booked to give a talk about the furore over whether he believed gay sex was a sin during the last general election.

The vote was put forward by pro-EU Conservative MPs who wanted to change the government’s post-Brexit strategy.

The MPs wanted the UK to join a customs union if it does not agree to a free-trade deal. However, the government won by a slight majority.

There were 12 Tory rebels in both votes, but the government’s total was boosted by four Labour MPs in the customs vote.

The customs union allows for tariff-free trading between member states, with a common tariff set for imports from the rest of the world.

The government did, however, lose a separate vote on its Trade Bill on the regulation of medicines after Brexit.

The UK is set to leave the EU in March 2019, but the two sides have yet been able to agree to a deal.

Earlier this month, David Davis resigned as Brexit Secretary. He has since been replaced by Dominic Raab, MP of Esher and Walton.