Boris Johnson has been warned that he risks sending the Conservatives into “terminal decline” if he allows this week’s party conference to become a rally for the hardest form of no-deal Brexit.

With a general election expected within weeks, former Tory MPs who were stripped of the party whip over Brexit said the prime minister must use to Manchester gathering to show that the party is more than a narrow ideological sect obsessed by Europe if he is to hold onto the voters who have previously delivered Conservative majorities.

The warnings came as ex-chancellor Philip Hammond said that the Tories’ “convulsions” over Brexit had left the party “unrecognisable” to him, with the broad church of earlier years replaced by “an ideological puritanism that brooks no dissent and is more and more strident in its tone”.

The conference opens just days after the Supreme Court’s humiliating ruling that Mr Johnson’s five-week suspension of parliament was unlawful. Unusually, the House of Commons will continue to sit during the four-day event, after MPs rejected the PM’s plea for a recess.

As a consequence, ministers will have to shuttle to and from London and opposition parties may take the opportunity to ambush the government, with the SNP calling for a vote of no confidence to instal an interim “unity government” while Tory MPs are away from Westminster.

Mr Johnson is under pressure over his conduct as mayor after he was referred to the police complaints body to assess whether he should face a criminal investigation over his links with American businesswoman Jennifer Arcuri.

Supreme Court verdict: Boris Johnson prorogation unlawful Show all 16 1 /16 Supreme Court verdict: Boris Johnson prorogation unlawful Supreme Court verdict: Boris Johnson prorogation unlawful The Supreme Court has ruled against Boris Johnson by declaring his government unlawfully shut down parliament for five weeks EPA Supreme Court verdict: Boris Johnson prorogation unlawful Lady Hale handed down the historic verdict, hailed by opposition MPs and anti-Brexit campaigners. She said all 11 judges were unanimous in deciding that the case is “justiciable”, so the government loses that part of the argument. “The court is bound to conclude therefore that the decision to advise Her Majesty to prorogue was unlawful because it had the effect of frustrating or preventing the ability of Parliament to carry out its constitutional functions.” EPA/Supreme Court Supreme Court verdict: Boris Johnson prorogation unlawful Gina Miller and her team react outside the Supreme Court are the verdict. She said: “Today is not a win for any individual or cause. It is a win for Parliamentary sovereignty, the separation of powers and independence of our British courts. Crucially, today’s ruling confirms that we are a nation governed by the rule of law, laws that everyone, even the Prime Minister, are subject to." PA Supreme Court verdict: Boris Johnson prorogation unlawful A person dressed as a caricature of British Prime Minister Boris Johnson in a prison uniform stands outside the Supreme Court AP Supreme Court verdict: Boris Johnson prorogation unlawful Labour leader Jeremy Corbyn called for Mr Johnson to “consider his position” following the landmark decision, while Lib Dem leader Jo Swinson said he was “not fit to be prime minister” Getty Supreme Court verdict: Boris Johnson prorogation unlawful Scottish National Party (SNP) Westminster leader Ian Blackford raises his arms as he comes outside. He said: "This is an absolutely stunning judgement by the Supreme Court today." He went on to say, "we all want to get back to work, and quite frankly, on the back of this, Boris Johnson must resign immediately." AFP/Getty Supreme Court verdict: Boris Johnson prorogation unlawful Crowds outside celebrated the verdict AP Supreme Court verdict: Boris Johnson prorogation unlawful Delegates at the Labour party conference applaud after hearing the news AFP/Getty Supreme Court verdict: Boris Johnson prorogation unlawful Speaker John Bercow said MPs must now “convene without delay” and confirmed the Commons would return at 11.30am on Wednesday AFP/Getty Supreme Court verdict: Boris Johnson prorogation unlawful Gina Miller said: "“As a result of this judgment, Parliament is open, it was never prorogued. I urge MPs to get back to work immediately.” AP Supreme Court verdict: Boris Johnson prorogation unlawful MPs Anna Soubry, Liz Saville Roberts and Caroline Lucas, together with SNP leader Ian Blackford, react. Green MP Caroline Lucas has said the Supreme Court’s decision is “just the start” Reuters Supreme Court verdict: Boris Johnson prorogation unlawful “The UK deserves a Prime Minister and a Government who act with honesty, integrity and in a manner consistent with our constitution, at all times." AFP/Getty Supreme Court verdict: Boris Johnson prorogation unlawful Anti-Brexit supporters react as they gather outside the Supreme Court in London, Tuesday, Sept. 24, 2019 as it makes it's decision on the legality of Prime Minister Boris Johnson's five-week suspension of Parliament. (AP Photo/Frank Augstein) Frank Augstein AP Supreme Court verdict: Boris Johnson prorogation unlawful The SNP’s Joanna Cherry QC says: “Boris Johnson’s position is untenable and he should have the guts to resign.” PA Supreme Court verdict: Boris Johnson prorogation unlawful epa07865617 Protesters react outside the Supreme Court after the result of a hearing on the prorogation of parliament, in London, Britain, 19 September 2019. The Supreme Court ruled that the suspension of parliament by British Prime Minister Boris Johnson was not lawful. EPA/NEIL HALL NEIL HALL EPA Supreme Court verdict: Boris Johnson prorogation unlawful Tom Tugendhat MP returned to the chamber in the House of Commons within minutes of the Supreme Court ruling PA

And Labour has demanded an independent conflict-of-interest inquiry into the prime minister, after Mr Hammond claimed he was backed by speculators who have bet billions of pounds on a no-deal Brexit.

Mr Hammond’s call for the PM to retrieve the party’s position by shifting to a position of compromise over Brexit was echoed by some of the other 21 MPs barred from being Tory candidates after they rebelled in a vote to prevent a no-deal outcome earlier this month.

Former foreign minister and deputy Tory chair Alistair Burt said the annual conference, which opens on Sunday, was a chance for the party to show that “it is about more than just Brexit”.

“It’s a question of to what extent this obsession with Europe has now become the defining characteristic of being a Conservative, to what extent has it truly entered the soul of the Conservative party?” he told The Independent.

Mr Johnson’s senior adviser and former Vote Leave supremo Dominic Cummings “has hijacked Number 10, but has he hijacked the party as well?” asked Mr Burt. “It really is important that the party demonstrates that its thinking goes way beyond Brexit and it is really thinking about what to do about schools and hospitals and the police, but also to what extent it can persuade the public that these things matter to it as much as leaving the EU.”

Former environment minister Richard Benyon said he was “greatly concerned” that Mr Johnson would allow the conference to become a Brexit rally.

Mr Benyon’s Newbury constituency in Berkshire is the kind of southern seat which ought to be safe for Tories, but may be vulnerable to Liberal Democrats because it voted Remain in the 2016 referendum.

“I’m hoping our fears will be proved wrong, but it depends what message he has decided to create.” Mr Benyon told The Independent.

“If he has a strategy about winning Brexit seats from Labour and not worrying too much about seats like mine or seats in London and Scotland, it’s a pretty drastic position. I’m hoping that’s not the case.”

Richard Benyon (Getty)

Calling on Mr Johnson to use the conference to show the party is “broad and inclusive”, Mr Benyon said: “It’s clearly the last conference before the election, so the message they send out is absolutely vital. All the work we did to reach out to groups who were no longer supporting the Conservatives pre-2005 could be thrown away. We can’t win elections appealing to just one sector of the population.”

Aberconwy MP Guto Bebb said it may already be too late for the Conservatives to recover their reputation among mainstream voters for whom Brexit is not the dominant issue.

“Over a number of years, the Conservative party has been hollowed out, the number of members is falling and the sensible people have walked away,” he said. “I’m sad to say that what I think we will see is that the Conservative party is in terminal decline.”

The former defence minister – who has announced he will not fight the next election – said he expected the conference, and Mr Johnson’s all-important keynote address, make Theresa May’s 2016 assault on the “citizens of nowhere” look like “a model of restraint”.

“I suspect the main thing we will see is the Conservative party continuing in the direction of becoming a new Brexit Party. I think it is their plan to whip up further division. I think the reputation of the Conservative party was trashed the moment they decided to endorse Boris Johnson as leader by such a margin.

“They have taken a decision which may deliver an election victory this year but in the long term means the party turning away from the pragmatic middle-of-the-road voters who would once have given them the benefit of the doubt because of their offer of economic competence and stability. These voters will be alienated by the ideological pursuit of Brexit by radicals and people with no stake in society.”

Former cabinet minister Justine Greening said it was vital for Mr Johnson to spell out in detail to activists the Brexit deal which he hopes to obtain from Brussels.

Justine Greening (Reuters)

“Conservative activists deserve to see what Boris Johnson’s detailed Brexit plan is,” Ms Greening told The Independent.

“He sold Britain Brexit alongside Michael Gove, now they’re in charge and Mr Johnson is prime minister, so what’s his plan?

“Theresa May published a detailed white paper, though it was the worst of all worlds on Brexit. Even if they disagreed with it, at least people knew what she wanted.”

A senior Tory source confirmed that the theme of the Manchester gathering would be to “get Brexit done”.

But he insisted the PM would demonstrate his focus on issues beyond Brexit by conducting visits to public services around the Manchester area throughout the conference.

In comments which appeared to confirm that Tories will fight the coming election on a “people versus parliament” platform, the source said: “The prime minister will be busy visiting our brilliant public servants and setting out ideas for dealing with the public’s priorities like the cost of living.

“Parliament will carry on blocking everything and trying to delay Brexit. This Parliament is now aimless – only voting to dither and delay – and the sooner it is dissolved the better.

“The country needs a new parliament, a new government and a new agenda so we can put the failure of the political class over the last three years behind us.