The former foreign secretary suggested the Prime Ministers so-called Chequers plan for leaving the EU amounted to a “ridiculous slipping away of our self-belief” instead of the “freedom” that was promised.
After arriving to a standing ovation in Birmingham, Johnson said his main fear is that Britain has “lost confidence in its basic beliefs in freedom” and accepted “foreign rule” by the European Union.
He also chided Labour and Jeremy Corbyn for failing to rule out a second Brexit referendum, saying another vote would subject the country to more “tedium, rancour and uncertainty”.
But in a wide-ranging speech, Johnson chose to focus on Brexit, issuing an explosive ultimatum for Mrs May to “chuck Chequers” and “deliver what the people voter for”.
He said exiting the EU on “Chequers terms” would be “politically humiliating” for Britain, leaving the nation “unable to make our own laws”.
BREXIT SPEECH: Boris Johnson said Theresa May's Brexit plan is a 'constitutional outrage'
(Pic: SKY NEWS)
In her Lancaster House speech in January last year, Mrs May said she would restore Britains sovereignty by taking the UK out of the customs union, single market and jurisdiction of the European Court of Justice.
He said claims “there is no other alternative” to Chequers should not be believed, arguing a Lancaster House-style Brexit would be a "win-win" for both Britain and the EU.
Johnson even won loud applause when he suggested that the PM risked being prosecuted under a 14th century law saying that "no foreign court or government shall have jurisdiction in this country".
He rejected as "total fantasy" the idea that it would be possible to "bodge" Brexit now and then negotiate a better deal after leaving in March 2019.
CHUCK CHEQUERS: Johnson has urged the PM to 'deliver what the people voted for'
In a scathing assessment of the PMs Chequers proposals, he fumed: “This is not democracy. That is not what we voted for.
“It is a constitutional outrage. Its not taking back control.”
In his closing gambit, Johnson urged the PM to revert back to the plan she put forward at Lancaster House, which would allow Britain to make an “elegant, dignified and graceful exit”.
He said: “And so for one last time, I urge our friends in government to deliver what the people voted for, to back Theresa May in the best way possible, by softly, quietly, and sensibly backing her original plan. And in so doing to believe in conservatism and to believe in Britain.
“Because if we get it wrong we will be punished. And if we get it right we can have a glorious future.
“This government will then be remembered for having done something brave and right and remarkable and in accordance with the wishes of the people.”
TAKE BACK CONTROL: Boris Johnson said 'there is time to chuck Chequers'
(Pic: SKY NEWS)
Johnson will pay only a flying one-day visit to a conference which has witnessed open warfare within the Tory party over Brexit, which critics say he has done nothing to calm.
He used a Sunday Times interview to describe Mrs May's own policy on EU withdrawal as "deranged" and "preposterous".
And he pointedly contrasted his record as the figurehead of the Leave campaign with that of the Remain-backing Mrs May, saying: "Unlike the Prime Minister, I fought for this."
Johnson last week urged the PM to “chuck Chequers” and instead pursue a “Canada-style” free trade deal with the EU in an essay outlining his vision for Brexit in the Telegraph.
FREEDOM: Johnson said a Chequers Brexit does not represent 'democracy'
The PM is expected to make major concessions in new proposals to the EU that could delay Brexit for years and restrict Britains ability to strike trade deals.
The Prime Minister is planning to keep Britain tied to European customs rules on goods beyond the scheduled leaving date of March 2019, according to the Times.
Under these plans her government would not have complete freedom to agree trade deals with nations across the world for several years after the transition period ends in December 2020, the paper claims.
Her Brexit proposals also reportedly involve accepting demands that goods entering Northern Ireland from Britain must meet European standards, meaning possible checks in the Irish Sea.
PACKED AUDIENCE: David Davis, former Brexit secretary, was among the audience members
“It is a constitutional outrage”
Speaking with BBC Radio 4s Today programme this morning, the PM said she is ready to present her new version of Chequers to Brussels.
"We will be setting forwards those proposals. Can I just explain why we are setting forward our own proposals? It is because the plan the European Union put forward is not acceptable to us," May told the BBC.
"Because the plan the European Union has put forward is one that effectively keeps Northern Ireland in the customs union and ensures that effectively there is a customs border down the Irish sea."
NEW PROPOSALS: The PM has said she will present her new plan to Brussels
However, European Commission president Jean-Claude Juncker warned Mrs May that there must be agreement on the Irish border if she wants a Brexit deal.
"When it comes to the border between the Republic of Ireland and Northern Ireland, we are sticking to the point of view we have expressed so many times – Ireland first,” Juncker said, addressing the European Parliament in Strasbourg.
His comments stress the importance Brussels places on solving the Irish border, an issue that has divided her party and raised the prospect of a no-deal.
The summit on October 18 to 19 has been billed as the "moment of truth" when it will become clear whether it is possible for the two sides to do a deal.