The prime minister survived a backbenchers' rebellion demanding a customs union deal with the EU - but lost a separate vote demanding free flow of medicines across Europe post-Brexit.
"I had to think who I wanted to see in the mirror for the rest of my life".
A strong rebellion on the Conservative benches could lead to an embarrassing defeat for a Government now assailed on all sides over its plans for leaving the European Union.
Twelve Conservative MPs rebelled against the government to vote for a proposal that would have kept the United Kingdom in the customs union if no alternative plan for frictionless trade with the EU had been agreed by January 2019. "We are cracking down right now on the European Union because they have not treated the United States fairly on trading".
But in the past 18 months, "it is as though a fog of self-doubt has descended", he said.
The government said being part of an EU customs union would prevent it from striking worldwide trade deals, won the vote by 307 to 301, a slim majority for May of just 6.
He said he was still undecided whether he would vote in favour of the Government's deal when it went before the House in its final form.
Brexiteers believe that keeps Britain too close to the European Union, while pro-Europeans think it fails to protect the country's dominant services sector, among other gripes.
Indeed, Mr Johnson claimed in the Commons that, under this plan, Britain was "volunteering for economic vassalage" and it would represent "Brexit in name only". "That was the vision that the Prime Minister rightly described a year ago".
Johnson's comments in that regard appear to echo those made by prominent Brexit figure Jacob Rees-Mogg, who watched on from the same bench as Johnson spoke, who had before described May's Brexit plan as "perpetual purgatory".
Johnson resigned from government rather than accept May's plan.
The thrust of Mr Johnson's address was that Mrs May had previously set out a "bold and ambitious" vision for Britain's future outside the European Union, before taking fright and diluting her Brexit plan.
"While the prime minister refuses to admit that her ill-judged red lines are a thing of the past, she has - at long last - set out a vaguely credible negotiating position", he said.
Johnson said the agreement reached by the Cabinet at Chequers would make the United Kingdom "rule takers", damaging its ability to strike new global trade deals.
Sources close to Mr Johnson said he had received permission from Speaker John Bercow to make the brief statement, amid intense speculation that he may use it to attack Theresa May's approach to Brexit.
Remainer conservative MPs warned that the Brexit plan amended by hardliners increase the likelihood of a no-deal divorce, and that prospect is becoming more tangible in Brussels as well.
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