The Liberal Democrats' chief whip says he "messed up" by allowing party leader Sir Vince Cable and his predecessor Tim Farron to skip Monday night's knife-edge vote on Brexit.
They could have cut the government's winning margin on the Customs Bill from three votes to just one.
Mr Farron said he had "called it wrong" and was sorry for what had happened.
And chief whip Alistair Carmichael said he had expected the vote to be "lost by hundreds".
In a statement tweeted by Sir Vince, Mr Carmichael said the government's winning margin should have been just one.
"By the time it became apparent that the vote was going to be close - it was too late to get two of our MPs, Vince and Tim, back in time to vote," he said.
"I'm taking responsibility and redoubling my efforts to stop Brexit."
I was authorised to be absent from the vote last night for a pre-arranged engagement. Nobody expected the vote to be as close as it was. We clearly called it wrong, as did Labour, and I take full responsibility for my part – the Tories don’t deserve any luck. pic.twitter.com/XoLVPawtmo— Tim Farron (@timfarron) July 17, 2018
A party source described Sir Vince's absence as a "bit unfortunate" and that he was elsewhere at a confidential political meeting "outside of the parliamentary estate".
Mr Farron was booked to give a talk, Illiberal Truths, about the furore over whether he believed gay sex was a sin during the last general election.
He tweeted that the Conservatives "don't deserve any luck".
All the party's other MPs - with the exception of new mum Jo Swinson who was 'paired' with an MP who didn't vote for the other side - cast their ballot against the government on amendments to legislation defining the UK's customs arrangements with the EU after it leaves in March 2019.
The amendments, tabled by Eurosceptic Tory MPs, were accepted by ministers - prompting a backlash by pro-European Tories, 14 of whom ended up voting against the government.
With Labour also voting against, the government scraped home in two votes by a margin of three.
The Lib Dems have been calling for a referendum on the final Brexit deal, with senior figures backing the cross-party People's Vote campaign.