When he became prime minister, Boris Johnson had a huge opportunity, and responsibility, to reunite our divided country. As the figurehead of the Leave campaign in the Brexit referendum, he was uniquely positioned to reach out to the 48% of us who voted to remain in the European Union and who have been ignored by the government ever since.

Johnson could have persuaded the most extreme Brexiters in his party that they should compromise with those who voted to remain. There are many young, urban and ethnic minority communities across Britain who are terrified about the future. Johnson could have chosen to use his position as the new prime minister to heal the divisions that have gripped us rather than widen them.

But the prime minister has chosen a far more dangerous path. He has made it clear that he intends to pursue a no-deal Brexit on 31 October, which would be catastrophic. He is refusing to meet EU leaders even to discuss doing a deal. Rather than straddling the divide in Britain, he has chosen to lead one side into battle against the other – and he will take no prisoners in the process.

This is the most extreme rightwing government anyone can remember, and its representatives more closely reflect the social beliefs held in the 1930s than they do modern Britain

It is well documented that Boris Johnson will say or do anything if he believes it is in his personal interest – including ignoring the best interests of the country. As a journalist, mayor of London and minister, he has never had any qualms about breaking his promises or bending the truth if it helps him there and then.

He has now decided that it is in his own personal interests to pursue an extreme hard Brexit whatever the cost, and to embrace the divisive, backward-looking and illiberal views of the hard-line rightwingers to whom he has handed top cabinet jobs. This is the most extreme rightwing government anyone can remember, and its representatives more closely reflect the social beliefs held in the 1930s than they do the views of modern and open Britain.

Downing Street is letting it be known that they are willing to use the most undemocratic and unconstitutional means possible to secure no deal – including riding roughshod over parliamentary sovereignty. So much for “taking back control”.

The speed at which a no-deal Brexit has moved from being unthinkable to extremely likely means that we must keep repeating that this would be a catastrophic outcome for Britain’s future. No leading figures on the leave side in the referendum campaign suggested such an extreme version of Brexit. It would make us poorer, now and for years to come. It would cause short-term and long-term disruption across the country. It could jeopardise the fragile Northern Ireland peace process. It would make us all less safe and secure, and diminish the way our country is seen around the world. And you don’t have to take my word for it – these points have been made by a wide range of people from the governor of the Bank of England to the most senior police officers.

It is a moot point whether Johnson truly shares the views of other populist far-right politicians around the world – from Trump to Orbán and Le Pen – but what is indisputable is that he has decided to adopt their playbook. They clearly view him as one of their own. There is a reason the US president described him as “Britain’s Trump”, and why former Trump aide Steve Bannon recently admitted he had been advising the new prime minister.

The former Trump aide Steve Bannon has admitted he has been advising Boris Johnson. Photograph: Sylvain Lefevre/Getty Images

This is a government whose leading lights include climate change deniers, people who have spent their whole lives attacking women’s equality and LGBTQ+ rights, and who are dedicated to the most-extreme form of Brexit. Remain-backing ministers were culled unless they signed up to backing no-deal. Whitehall departments have been stuffed with hard-line Brexiter ministers and political special advisers.

The urgent question now facing people like me, who believe that a no-deal Brexit must be stopped at all costs, is what we can do to prevent this government from driving us over the no-deal cliff edge. This is our last chance and the odds are stacked against us, but we are not without hope – the Tory majority has been reduced to just one, and increasing numbers of Tory MPs share our horror at just how extreme Johnson’s administration is turning out to be.

But there is no time for parliamentary poker, no time to bluff. You’re either committed to stopping no deal – the default outcome – or you’re not. We can’t trust the Johnson government not to play fast and loose with our country’s future, so MPs must take decisive action to take power out of the government’s hands and give it back to the British people in the form of an election or people’s vote before any irreversible decisions are taken.

The first step must be an immediate vote of no confidence in Johnson’s government as soon as parliament returns in September – which I urge London MPs from all parties to back, in the interests of our city and our nation. We can’t afford any delay – which would risk the issue being taken out of parliament’s hands.

I will give my full support to any efforts required to stop an undemocratic no deal – whether they be a general election, taking control of the parliamentary order paper, or even a temporary government of national unity, if, as many constitutional experts are now saying, this is the only way to stop Johnson and his catastrophic no-deal Brexit for sure. The starting point for any conversations about an alternative government that can command the confidence of parliament must be that it would be led by Jeremy Corbyn as the leader of the opposition and by far the biggest party opposed to no deal.

This is not about party politics – it’s far bigger than that. It’s about our expectation that the British government will serve our national interest and observe our most cherished democratic traditions. Britain is an open, liberal and outward-looking democracy – and we must keep it that way. That’s why I will support doing whatever it takes under the law over the weeks ahead to stop Boris Johnson from crashing Britain out of the EU without a deal, and to stop him inflicting the views of the most extreme elements of his party on the British people.

Sadiq Khan is mayor of London