BORIS Johnson has swung behind Theresa May, insisting the whole Cabinet is now united in support of her approach to Brexit as he issued a positive plea to “let the lion roar”.

With the Conservative conference dominated by the issue of the Prime Minister’s leadership and what many Tories believe to be the Foreign Secretary’s disloyalty to her, Mr Johnson made a point of praising his party leader, telling activists that Labour under Jeremy Corbyn had not won the General Election but “you won - we won - Theresa May won”.

He went on: "She won more votes than any party leader and took this party to its highest share of the vote in any election in the last 25 years and the whole country owes her a debt for her steadfastness in taking Britain forward, as she will, to a great Brexit deal.

"Based on that Florence speech on whose every syllable, I can tell you the whole Cabinet is united," declared the Foreign Secretary.

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In a round of media interviews on the eve of her keynote speech, Mrs May – who was not in the conference hall for Mr Johnson’s speech - insisted she was not being "undermined" by her colleague, noting that a true leader did not surround herself with “yes men” but with diverse voices.

And this morning in her conference address, the PM will strike a defiant pose with a message to her doubters that she will not stand aside.

Mrs May is expected to say: “It has never been my style to hide from a challenge, to shrink from a task, to retreat in the face of difficulty, to give up and turn away.

"And it is when tested the most that we reach deep within ourselves and find that our capacity to rise to the challenge before us may well be limitless.”

Senior party sources have suggested that Cabinet colleagues of Mr Johnson are “fed up” with his outbursts on Brexit, which, they believe are designed to box the PM into a hard Brexit.

One suggested that “sooner rather than later” he expected the Secretary of State to resign from the Cabinet, not so much to seek to bring his leader down but to position himself as a candidate with clear deep blue water once Mrs May does step aside, which many senior Tories believe, despite the PM’s defiant conference words, will be after Britain leaves the EU in March 2019.

A major theme of Mr Johnson’s speech was about taking on the “semi-Marxism” of Mr Corbyn’s Labour Party, warning about the dangers of a retreat to the socialism of the 1970s and exhorting his party to make the arguments anew supporting free market capitalism to “win the future”.

However, the core message of his address was to be upbeat about the positive effects of Brexit and the prospect of a truly global Britain.

"There is a huge desire out there for us to engage with the world more emphatically than ever before,” he declared.

"And after Brexit that is what our partners are going to get as this country is freed from endlessly trying to block things in Brussels committee rooms, freed to stop being negative and to start being positive about what we believe in, including free trade.”

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The Foreign Secretary argued it was “manifestly absurd” to argue that European values, culture or civilisation were somehow defined by the institutions of the EU.

“We will be no less European[after Brexit]. Britain will continue to be European in culture, geography, history, architecture, spiritually, morally, you name it."

He insisted the Conservatives could win the future because they were the party, which believed in Britain and in the potential of its people and were privileged collectively to be placed in charge of “this amazing country at a critical moment in our history”.

He added the party was not the lion; rather, the country was. “But it is up to us now…to let that lion roar."