Theresa May last night declared war on Boris Johnson after allies said they had rumbled a plot by her Election guru to install the former Foreign Secretary as the next Prime Minister.

Senior figures at Tory HQ claim that Sir Lynton Crosby is behind plans to mount a nationwide campaign against Mrs May’s Chequers agreement on Brexit as the precursor to a Boris leadership challenge.

Australian-born Sir Lynton, who masterminded Mr Johnson’s London mayoral victory in 2008 and who remains a close friend, is said to be motivated by ‘revenge’ after No 10 blamed the strategist for last year’s botched General Election.

Mr Johnson denies plotting with Sir Lynton to derail Mrs May’s Brexit negotiations and seize Downing Street. But in a sharply worded shot across his bows last night, a senior Tory source told The Mail on Sunday: ‘Boris hasn’t thought this through. His plan could result in us delaying leaving the EU, or even not leaving at all. If that happens, the party membership would never forgive him.’

Allies of Theresa May said they had rumbled a plot by her election guru to install former Foreign Secretary Boris Johnson (pictured with Lynton Crosby) as the next Prime Minister

The row comes as the Prime Minister last night vowed not to make any compromises to the Chequers deal, nor to ‘give in’ to demands for a second referendum – branding it a ‘gross betrayal’ of voters’ trust.

She said of the millions who voted in the original referendum: ‘They trusted that their vote would count; that after years of feeling ignored by politics, their voices would be heard. To ask the question all over again would be a gross betrayal of our democracy – and a betrayal of that trust.’

Mr Johnson’s popularity among Tory Party members has soared since he resigned as Foreign Secretary in July in protest at Mrs May’s Chequers plan, which would see the UK agreeing to follow EU regulations on trade, the environment and social affairs. He said it left the country at risk of ‘economic vassalage’ and accused Mrs May of ‘dithering’ over the UK’s strategy.

Meanwhile, Mrs May’s allies say that executives working for Sir Lynton’s powerful campaign company, CTF Partners, have been in discussions to ‘put rocket boosters’ under Change Britain, a group set up to argue for a hard Brexit, and turn it into a guerilla campaign against the Chequers deal.

They fear that the group will mount rallies around the country headed by Mr Johnson and other high-profile Brexiteers. Change Britain has strong links with the influential European Research Group (ERG) of Brexiteer Tory MPs led by Jacob Rees-Mogg.

Mrs May’s allies believe that Sir Lynton’s outfit has been targeting Eurosceptic Tory donors to fund the campaign.

They are also suspicious that Mr Jonson seems to have been ‘singing from the Lynton hymn sheet’ since his resignation – and point to the furore over Mr Johnson’s inflammatory claim that burkas make women look like ‘bank robbers’ or ‘letterboxes’. Sir Lynton, 62, is an exponent of so-called ‘dog-whistle politics’, in which controversial messages, couched in relatively inoffensive language, are targeted at the Hard-Right.

The Prime Minister (pictured in Cape Town) last night vowed not to make any compromises to the Chequers deal, nor to ‘give in’ to demands for a second referendum

Sir Lynton, dubbed the ‘Wizard of Oz’ for his previous successes, lost some of his lustre after playing a key role in the disastrous 2017 Election campaign which led to the loss of Mrs May’s Commons majority.

The row comes as MPs return to the Commons after the summer recess to prepare for a series of bruising battles over Brexit, with Mrs May hoping to strike a final deal with the EU within the next two months and then win Parliament’s approval. But she faces opposition from dozens of Tory MPs in the ERG, who share Mr Johnson’s objections to the Chequers deal.

It has led to intense speculation among MPs that if Mrs May tries to force Chequers through, then the ERG would force a vote of no confidence in her. If 48 MPs make such a call, then a vote would be held, triggering a leadership contest if Mrs May lost.

A source close to Mr Johnson said: ‘Boris has always backed the aims of Change Britain but he has no role in the organisation.’

A spokesman for CTF said that while Sir Lynton had not personally ‘spoken with nor met anyone from Change Britain’, the company’s senior staff ‘were often consulted because of their campaign expertise and knowledge’.

He added: ‘The better the deal the Prime Minister secures in Brexit, the better the prospects for the Conservatives at the next Election.’

SIMON WALTERS: Revenge of 'Wizard of Oz' Sir Lynton Crosby blamed for Theresa May's election humiliation

The feud between Sir Lynton Crosby and Theresa May dates back to her disastrous decision to call a snap election in June 2017.

Both are said to blame each other for the fiasco.

When Australian-born Crosby first heard of the snap poll from a May aide, he was in Fiji at a family gathering and his stunned response was: ‘I’m not sure that’s a smart idea, mate.’

The Mail on Sunday revealed last year how he sent Mrs May a ‘killer memo’ warning her it could backfire. He said there was ‘a lot of risk’ in calling a surprise ballot: voters were desperate to ‘avoid uncertainty’– but by going to the polls, she was doing the opposite.

Instead of the landslide win she expected, she could do no better than David Cameron’s narrow win in 2015, Crosby advised. In the event she did worse, and lost her Commons majority.

For their part, Mrs May’s supporters said she blamed him for key campaign mistakes.

They blamed Crosby for ordering her to run a presidential style ‘strong and stable’ campaign which collapsed days after the manifesto launch when she was forced to tear up plans for a so-called ‘dementia tax.’ Sir Lynton also fell out with Mrs May’s joint chief of staff Nick Timothy, with Timothy subsequently accusing him of failing to spot Jeremy Corbyn’s support among young voters.

Theresa May and election guru Lynton Crosby, pictured, are said to blame each other for the Conservatives' failure to win a majority at the snap election in June 2017

Tough-talking Crosby is known as the ‘Wizard of Oz’ because of his record of election campaign successes.

He helped Right-wing Australian politician John Howard win four elections and secured two victories in London mayoral elections for Boris Johnson.

He was reputedly paid £500,000 a year to advise Mr Cameron and helped him pull off a surprise victory over Ed Miliband in 2015.

But his campaigning style is not to everyone’s taste. He is credited with inventing ‘dog whistle’ tactics, whereby politicians echo shrill sentiments on populist issues such as immigration, without actually endorsing them.

This newspaper disclosed in 2012 how he told Mr Johnson not to bother with ‘f****** Muslims’ in his mayoral campaign, which was clearly echoed in the former Foreign Secretary’s controversial recent comments about the burka.

Sir Lynton also once boasted of getting Australian PM Mr Howard elected despite privately calling him ‘as dull as bats***’.

He calls his no-nonsense political style ‘getting the barnacles off the boat’ – to offer voters a streamlined message.