Boris Johnson has joked that the Prime Minister could be prosecuted for handing over British sovereignty to Brussels in a direct challenge to her authority on the eve of her party conference speech.
Mr Johnson demonstrated his power over the Conservative membership, attacking Theresa May’s plan for Brexit in an address that drew around 1,000 delegates at the party’s conference in Birmingham.
The former Foreign Secretary set out to prove that he was the only source of energy and authority in Birmingham, filling the largest meeting hall at the conference centre and setting out a domestic policy agenda that attacked Mrs May’s record and that of her government.
But his greatest criticism was reserved for the Prime Minister’s Chequers plan to keep the UK in a free trade area for goods with the EU after Brexit, which Mr Johnson called an “democratic outrage”.
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"It occurs to me that the authors of the Chequers proposal risk prosecution under the 14th-century statute of praemunire, which says that no foreign court or government shall have jurisdiction in this country," he said at one point.
Mr Johnson said Chequers would leave the UK "locked in the tractor beam of Brussels", and warned that the only winners from a Chequers-style Brexit would be the far-right and far-left in British politics.
And he urged Tory delegates to persuade the Prime Minister to "chuck Chequers" and return to the hard Brexit blueprint she first set out in her Lancaster House speech, when she said she would take the UK out of the customs union, single market and jurisdiction of the European Court of Justice.
Speaking on the fringe of the conference, the former foreign secretary - who quit Mrs May's Cabinet in July in protest at the Chequers plan - won an enthusiastic reaction on a scale not seen so far inside the main hall.
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.
And he warned that if a deal based on Chequers was agreed with the EU, it would "embolden" those calling for a second referendum - something he said would be "disastrous for trust in politics".
Mr Johnson said that Mrs May's blueprint - which ties Britain to a common rulebook with the EU for trade in goods - would be "politically humiliating for a £2 trillion economy" and would prevent the UK from making its own laws and subject it to the directives of Brussels.
"This is not pragmatic, it is not a compromise. It is dangerous and unstable - politically and economically," he said.
"My fellow Conservatives, this is not democracy. This is not what we voted for. This is an outrage.
"This is not taking back control: this is forfeiting control."
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And he warned: "If we get it wrong - if we bottle Brexit now - believe me, the people of this country will find it hard to forgive.
He was loudly clapped and cheered as he said: "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.”