Jo Johnson dramatically quit as transport minister, branding Theresa May’s Brexit plan a “con” and demanding a Final Say referendum to let the public decide.

The brother of Boris Johnson – a Remain supporter – described the prime minister’s negotiations as a “failure of British statecraft on a scale unseen since the Suez crisis”.

In a devastating indictment of Ms May’s strategy, he accused her of planning to trap the UK in a “boundless transitionary period” for years to come, with key controversies undecided.

Warning Britain “stands on the brink of the greatest crisis since the Second World War”, Mr Johnson added: “The democratic thing to do is to give the public the final say.”

The call comes after the number of signatures on The Independent’s petition for a Final Say referendum cleared one million, after just three months.

The resignation was not immediately followed by any other ministerial walkouts, but was a serious blow to Ms May’s hopes of a smooth path to an agreement at home and abroad next week.

Worse, it came as the Democratic Unionist Party (DUP), which props the Tories up in parliament, stepped up its threats to vote against it over the Irish border “backstop”.

It accused the prime minister of betraying her promise not to sign a withdrawal deal that could open the door to a customs border in the Irish sea.

A historic moment: thousands march to demand Final Say on Brexit Show all 65 1 /65 A historic moment: thousands march to demand Final Say on Brexit A historic moment: thousands march to demand Final Say on Brexit An estimated 700,000 people marched through London to demand a final say on the withdrawal agreement Angela Christofilou/The Independent A historic moment: thousands march to demand Final Say on Brexit Red smoke from a canister hangs in the air as around 100,000 demonstrators march through London during a People's Vote anti-brexit demonstration savings banners and placards Anti-Brexit People's Vote March for the Future in London Rex A historic moment: thousands march to demand Final Say on Brexit Final Say march for the future London, 20 October 2018 Credit: Angela Christofilou/The Independent Angela Christofilou/The Independent Angela Christofilou/The Independent A historic moment: thousands march to demand Final Say on Brexit Mayor of London Sadiq Khan takes part in the People's Vote March for the Future in London, a march and rally in support of a second EU referendum. PA A historic moment: thousands march to demand Final Say on Brexit Final Say campaigners take part in the peoples vote march for the future in London 20/10/2018 Protesters wearing final Say shirts and holding placards Angela Christofilou A historic moment: thousands march to demand Final Say on Brexit Final Say march for the future London, 20 October 2018 Credit: Angela Christofilou/The Independent Angela Christofilou/The Independent Angela Christofilou/The Independent A historic moment: thousands march to demand Final Say on Brexit Final say Brexit march - final say on our furture London, 20 October 2018 Angela Christofilou/The Independent Angela Christofilou/The Independent A historic moment: thousands march to demand Final Say on Brexit Final Say march for the future London, 20 October 2018 Credit: Angela Christofilou/The Independent Angela Christofilou/The Independent Angela Christofilou/The Independent A historic moment: thousands march to demand Final Say on Brexit MP Chuka Umunna (left) and MP Vince Cable (right) as MP Anna Soubry (centre) addresses Anti-Brexit campaigners at a rally after the People's Vote March for the Future in London, a march and rally in support of a second EU referendum. PA A historic moment: thousands march to demand Final Say on Brexit Final Say march for the future London, 20 October 2018 Credit: Angela Christofilou/The Independent Angela Christofilou/The Independent Angela Christofilou/The Independent A historic moment: thousands march to demand Final Say on Brexit Final Say march for the future London, 20 October 2018 Credit: Angela Christofilou/The Independent Angela Christofilou/The Independent Angela Christofilou/The Independent A historic moment: thousands march to demand Final Say on Brexit A People’s Vote march attendee calls for a Final Say Angela Christofilou A historic moment: thousands march to demand Final Say on Brexit Demonstrators with banners 'We're with EU' during the People's Vote March for the Future in London, Britain, 20 October 2018. Reports state that the 'March for the Future' is to be led by a column of young people and call for a Peopleâ€™s Vote on the Brexit deal. After marching through central London, there will be a rally on stage in Parliament Square, including speeches from Mayor of London Sadiq Khan. EPA A historic moment: thousands march to demand Final Say on Brexit Final Say march for the future London, 20 October 2018 Credit: Angela Christofilou/The Independent Angela Christofilou/The Independent Angela Christofilou/The Independent A historic moment: thousands march to demand Final Say on Brexit Final Say march for the future London, 20 October 2018 Credit: Angela Christofilou/The Independent Angela Christofilou/The Independent Angela Christofilou/The Independent A historic moment: thousands march to demand Final Say on Brexit Final Say march for the future London, 20 October 2018 Credit: Angela Christofilou/The Independent Angela Christofilou/The Independent Angela Christofilou/The Independent A historic moment: thousands march to demand Final Say on Brexit Protesters at London’s march for the future in October The Independent A historic moment: thousands march to demand Final Say on Brexit Editor of The Independent Christian Broughton speaks to demonstrators in Parliament Sqaure after they take part in a march calling for a People's Vote on the final Brexit deal, in central London on October 20, 2018. - Britons dreading life outside Europe gathered from all corners of the UK to London on Saturday to try to stop their country's looming breakup with the EU. AFP/Getty A historic moment: thousands march to demand Final Say on Brexit A poster at the March for the Future in October The Independent A historic moment: thousands march to demand Final Say on Brexit Anti-Brexit campaigners take part in the People's Vote March for the Future in London, a march and rally in support of a second EU referendum. PA A historic moment: thousands march to demand Final Say on Brexit Final Say march for the future London, 20 October 2018 Credit: Angela Christofilou/The Independent Angela Christofilou/The Independent Angela Christofilou/The Independent A historic moment: thousands march to demand Final Say on Brexit Final Say campaigners take part in the peoples vote march for the future in London 20/10/2018 Angela Christofilou A historic moment: thousands march to demand Final Say on Brexit Final say Brexit march - final say on our furture London, 20 October 2018 Angela Christofilou/The Independent Angela Christofilou/The Independent A historic moment: thousands march to demand Final Say on Brexit Final say Brexit march - final say on our furture London, 20 October 2018 Angela Christofilou/The Independent Angela Christofilou/The Independent A historic moment: thousands march to demand Final Say on Brexit Anti-Brexit campaigners take part in the People's Vote March for the Future in London, a march and rally in support of a second EU referendum. PA A historic moment: thousands march to demand Final Say on Brexit Final Say march for the future London, 20 October 2018 Credit: Angela Christofilou/The Independent Angela Christofilou/The Independent Angela Christofilou/The Independent A historic moment: thousands march to demand Final Say on Brexit Final say Brexit march - final say on our furture London, 20 October 2018 Angela Christofilou/The Independent Angela Christofilou/The Independent A historic moment: thousands march to demand Final Say on Brexit Final Say march for the future London, 20 October 2018 Credit: Angela Christofilou/The Independent Angela Christofilou/The Independent Angela Christofilou/The Independent A historic moment: thousands march to demand Final Say on Brexit Final Say march for the future London, 20 October 2018 Credit: Angela Christofilou/The Independent Angela Christofilou/The Independent Angela Christofilou/The Independent A historic moment: thousands march to demand Final Say on Brexit Final Say march for the future London, 20 October 2018 Credit: Angela Christofilou/The Independent Angela Christofilou/The Independent Angela Christofilou/The Independent A historic moment: thousands march to demand Final Say on Brexit Final Say march for the future London, 20 October 2018 Credit: Angela Christofilou/The Independent Angela Christofilou/The Independent Angela Christofilou/The Independent A historic moment: thousands march to demand Final Say on Brexit Final Say march for the future London, 20 October 2018 Credit: Angela Christofilou/The Independent Angela Christofilou/The Independent Angela Christofilou/The Independent A historic moment: thousands march to demand Final Say on Brexit Demonstrators hold placards as they take part in a march calling for a People's Vote on the final Brexit deal, in central London on October 20, 2018. - Britons dreading life outside Europe gathered from all corners of the UK to London on Saturday to try to stop their country's looming breakup with the EU. AFP/Getty A historic moment: thousands march to demand Final Say on Brexit Final Say march for the future London, 20 October 2018 Credit: Angela Christofilou/The Independent Angela Christofilou/The Independent Angela Christofilou/The Independent A historic moment: thousands march to demand Final Say on Brexit Final Say march for the future London, 20 October 2018 Credit: Angela Christofilou/The Independent Angela Christofilou/The Independent Angela Christofilou/The Independent A historic moment: thousands march to demand Final Say on Brexit Final Say march for the future London, 20 October 2018 Credit: Angela Christofilou/The Independent Angela Christofilou/The Independent Angela Christofilou/The Independent A historic moment: thousands march to demand Final Say on Brexit Final say Brexit march - final say on our furture London, 20 October 2018 Angela Christofilou/The Independent Angela Christofilou/The Independent A historic moment: thousands march to demand Final Say on Brexit Final say Brexit march - final say on our furture London, 20 October 2018 Angela Christofilou/The Independent Angela Christofilou/The Independent A historic moment: thousands march to demand Final Say on Brexit Final Say march for the future London, 20 October 2018 Credit: Angela Christofilou/The Independent Angela Christofilou/The Independent Angela Christofilou A historic moment: thousands march to demand Final Say on Brexit Final say Brexit march - final say on our furture London, 20 October 2018 Angela Christofilou/The Independent Angela Christofilou/The Independent A historic moment: thousands march to demand Final Say on Brexit Final Say march for the future London, 20 October 2018 Credit: Angela Christofilou/The Independent Angela Christofilou/The Independent Angela Christofilou/The Independent A historic moment: thousands march to demand Final Say on Brexit Final Say march for the future London, 20 October 2018 Credit: Angela Christofilou/The Independent Angela Christofilou/The Independent Angela Christofilou/The Independent A historic moment: thousands march to demand Final Say on Brexit Final say Brexit march - final say on our furture London, 20 October 2018 Angela Christofilou/The Independent Angela Christofilou/The Independent A historic moment: thousands march to demand Final Say on Brexit Final Say march for the future London, 20 October 2018 Credit: Angela Christofilou/The Independent Angela Christofilou/The Independent Angela Christofilou/The Independent A historic moment: thousands march to demand Final Say on Brexit Final Say march for the future London, 20 October 2018 Credit: Angela Christofilou/The Independent Angela Christofilou/The Independent Angela Christofilou/The Independent A historic moment: thousands march to demand Final Say on Brexit Final Say march for the future London, 20 October 2018 Credit: Angela Christofilou/The Independent Angela Christofilou/The Independent Angela Christofilou/The Independent A historic moment: thousands march to demand Final Say on Brexit Final say Brexit march - final say on our furture London, 20 October 2018 Angela Christofilou/The Independent Angela Christofilou/The Independent A historic moment: thousands march to demand Final Say on Brexit Final Say march for the future London, 20 October 2018 Credit: Angela Christofilou/The Independent Angela Christofilou/The Independent Angela Christofilou/The Independent A historic moment: thousands march to demand Final Say on Brexit Final say Brexit march - final say on our furture London, 20 October 2018 Angela Christofilou/The Independent Angela Christofilou/The Independent A historic moment: thousands march to demand Final Say on Brexit Final Say march for the future London, 20 October 2018 Credit: Angela Christofilou/The Independent Angela Christofilou/The Independent Angela Christofilou/The Independent A historic moment: thousands march to demand Final Say on Brexit Final Say march for the future London, 20 October 2018 Credit: Angela Christofilou/The Independent Angela Christofilou/The Independent Angela Christofilou A historic moment: thousands march to demand Final Say on Brexit Final Say campaigners take part in the peoples vote march for the future in London 20/10/2018 campaigner wrapped in EU flag Angela Christofilou A historic moment: thousands march to demand Final Say on Brexit Final Say march for the future London, 20 October 2018 Credit: Angela Christofilou/The Independent Angela Christofilou/The Independent Angela Christofilou/The Independent A historic moment: thousands march to demand Final Say on Brexit Final Say march for the future London, 20 October 2018 Credit: Angela Christofilou/The Independent Angela Christofilou/The Independent Angela Christofilou/The Independent A historic moment: thousands march to demand Final Say on Brexit Final Say campaigners take part in the peoples vote march for the future in London 20/10/2018 Angela Christofilou A historic moment: thousands march to demand Final Say on Brexit Demonstrators pass Trafalgar Square as they take part in a march calling for a People's Vote on the final Brexit deal, in central London on October 20, 2018. - Britons dreading life outside Europe gathered from all corners of the UK to London on Saturday to try to stop their country's looming breakup with the EU. AFP/Getty A historic moment: thousands march to demand Final Say on Brexit Demonstrators wave Union and European flags and hold up placards as they pass Trafalgar Square, taking part in a march calling for a People's Vote on the final Brexit deal, in central London on October 20, 2018. - Britons dreading life outside Europe gathered from all corners of the UK to London on Saturday to try to stop their country's looming breakup with the EU. AFP/Getty A historic moment: thousands march to demand Final Say on Brexit Demonstrators hold placards as they take part in a march calling for a People's Vote on the final Brexit deal, in central London on October 20, 2018. - Britons dreading life outside Europe gathered from all corners of the UK to London on Saturday to try to stop their country's looming breakup with the EU. AFP/Getty A historic moment: thousands march to demand Final Say on Brexit A demonstrator holds a message during a march calling for a People's Vote on the final Brexit deal, in central London on October 20, 2018. - Britons dreading life outside Europe gathered from all corners of the UK to London on Saturday to try to stop their country's looming breakup with the EU. AFP/Getty A historic moment: thousands march to demand Final Say on Brexit Anti-Brexit campaigners take part in the People's Vote March for the Future in London, a march and rally in support of a second EU referendum. PA A historic moment: thousands march to demand Final Say on Brexit Demonstrators take part in the 'People's Vote March for the Future,' in central London, Britain, 20 October 2018. Reports state that the 'March for the Future' is to be led by a column of young people and call for a Peopleâ€™s Vote on the Brexit deal. After marching through central London, there will be a rally on stage in Parliament Square, including speeches from Mayor of London Sadiq Khan EPA A historic moment: thousands march to demand Final Say on Brexit A protester brandishes an Independent t-shirt during the Brexit March Angela Christofilou A historic moment: thousands march to demand Final Say on Brexit Anti-Brexit campaigners take part in the People's Vote March for the Future in London, a march and rally in support of a second EU referendum PA A historic moment: thousands march to demand Final Say on Brexit Tens of thousands of people take part in People's Vote March for the Future in central London. The march organised by the People's Vote campaign is led by young people calling for a People's Vote on the Brexit deal Rex A historic moment: thousands march to demand Final Say on Brexit Anti-Brexit campaigners take part in the People's Vote March for the Future in London, a march and rally in support of a second EU referendum PA

A leaked letter from Ms May said she would not allow a break-up of the UK customs territory to “come into force” – implying it would, nevertheless, be in the agreement.

Arlene Foster, the DUP leader, reacted angrily, saying: “No unionist would be able to support that.

“In other words, we [Northern Ireland] will have a different regulatory system from the rest of the United Kingdom, and essentially there's going to be a border down the Irish sea.”

In a lengthy article, Mr Johnson said it was “increasingly clear” that the withdrawal deal “will be a terrible mistake”, saying: “The choice being presented to the British people is no choice at all.

“The first option is the one the government is proposing: an agreement that will leave our country economically weakened, with no say in the EU rules it must follow and years of uncertainty for business.

“The second option is a no-deal Brexit that I know as a transport minister will inflict untold damage on our nation.

“To present the nation with a choice between two deeply unattractive outcomes, vassalage and chaos, is a failure of British statecraft on a scale unseen since the Suez crisis.”

Mr Johnson became the ninth Tory MP to back a fresh referendum. He wrote: “Given that the reality of Brexit has turned out to be so far from what was once promised, the democratic thing to do is to give the public the final say.

“This would not be about rerunning the 2016 referendum, but about asking people whether they want to go ahead with Brexit now that we know the deal that is actually available to us, whether we should leave without any deal at all or whether people on balance would rather stick with the deal we already have inside the European Union.”

The resignation immediately drew praise from Boris Johnson – even though the pair walked out of the government from opposite sides of the Brexit controversy.

“Boundless admiration as ever for my brother Jo,” the former foreign secretary tweeted. “We may not have agreed about Brexit but we are united in dismay at the intellectually and politically indefensible of the UK position.”

Conservative Party supporters of a Final Say referendum were quick to react. Heidi Allen tweeted: “Hats off to @JoJohnsonUK , putting his country first. If we don’t know what our future trade deal with the EU will look like, MPs shouldn’t support it.”

Anna Soubry tweeted: “Huge respect for @JoJohnsonUK. It’s tough resigning from a ministerial post, he’s done the right thing. Now is the time for people to stand up for what they believe in or we will sleepwalk to a #Brexit disaster.

And Justine Greening said: “Well done to @JoJohnsonUK on standing up for what he believes in. He was a fantastic minister to work with whilst we were at the DfE. It’s a sad loss for Gvt, but Brexit is above party politics.”

Jenny Chapman, Labour’s shadow Brexit minister, said Mr Johnson was the 18th minister to quit Ms May’s government, adding: “She has lost all authority and is incapable of negotiating a Brexit deal within her own party, let alone with the EU.”

Downing Street issued a short, icy statement, saying: “The referendum in 2016 was the biggest democratic exercise in this country’s history.

“We will not under any circumstances have a second referendum. The prime minister thanks Jo Johnson for his work in government.”