By prevailing in Tuesday's vote, she avoided for now the prospect of having to go back on her word that Britain will not be part of any customs union after leaving the EU - something that would have enraged the pro-Brexit wing of her party.
But he warned in the 18 months since that speech, a "fog of self-doubt" has fallen over Britain's stance.
Speculation among political commentators in London is that around 40 MPs are thought to have told the chairman of the Conservative backbench committee that they have lost confidence in May.
However Mrs May rejected claims from MPs that a "no-deal" outcome was becoming more likely following the row over her Brexit plans this week.
May's vulnerability in parliament, where she lost her majority in an ill-judged election a year ago, was laid bare on Monday and Tuesday when she faced rebellions from both the pro-Brexit and pro-EU wings of her party.
He added: 'All it shows is that, while the auto is careering towards the Brexit cliff-edge, senior Conservatives are fighting over what song to play on the stereo.' The prime minister wasn't even in the Commons to hear the address because she was answering questions about Brexit to a committee of MPs. For his allies, that was the point.
This week Ms. May agreed to modify the plan slightly but that only angered the soft Brexit backers in her caucus who supported the customs arrangement and said she was caving in to the Brexiteers.
Pro-EU lawmakers have tabled a change to the wording of the bill to try to force the government to pursue a customs union with the EU if ministers fail to agree an agreement which establishes "a frictionless free trade area for goods".
However, the parties still have differences on a few key issues, particularly regarding the border with Northern Ireland.
Barnier said Britain shared the EU's desire for an ambitious trading relationship, a level playing field and a security partnership.
With the ruling Conservative party deeply divided over Brexit, Johnson said he backed the Prime Minister but savaged her Brexit policy saying it would leave Britain in a "miserable, permanent limbo".
Only 16 per cent of voters say May is handling the Brexit negotiations well, compared with 34 percent who say that Johnson would do a better job, according to the poll conducted by YouGov for The Sunday Times newspaper.
Mr Johnson's speech was re-tweeted by a host of prominent Brexiteer Tories, among them Jacob Rees-Mogg, chair of the influential European Research Group of MPs.
On Monday, May infuriated lawmakers who wanted to keep the closest possible ties with the European Union when she made a decision to accept a number of demands by hardline pro-Brexit members.
Britain is set to leave the bloc on March 30, but the two sides want to strike the divorce agreement by late October in order to give parliaments enough time to endorse a deal. It falls eight short of the 48 needed to trigger a vote.
But even calling a vote would harm the Conservative party and could pave the way for a snap general election.