Theresa May will call on the British people to unite as she triggers article 50, beginning a two-year process that will see the UK leave the European Union and sever a political relationship that has lasted 44 years.
A letter signed by the prime minister will be hand-delivered to the president of the European council at about 12.30pm – as she rises in Westminster to deliver a statement to MPs signalling the end of the UK’s most significant diplomatic association since the end of the second world war.
May will aim to strike a note of reconciliation when she addresses the Commons, claiming this is the time for Brexiters and remainers to “come together” after holding an early-morning meeting of her cabinet.
“When I sit around the negotiating table in the months ahead, I will represent every person in the whole United Kingdom – young and old, rich and poor, city, town, country and all the villages and hamlets in between. And yes, those EU nationals who have made this country their home,” she will say.
Labour said it respected the decision of the British public but vowed to hold the government to account. Jeremy Corbyn, the Labour leader, said: “Britain is going to change as a result. The question is how … It will be a national failure of historic proportions if the prime minister comes back from Brussels without having secured protection for jobs and living standards.”
But the historic action that formally begins the Brexit process, following last June’s referendum, continues to pitch senior political figures against each other as the ferocity of the debate shows no sign of abating.
Michael Heseltine, the Conservative former cabinet minister, told the Guardian the move represented the “worst peacetime decision taken by any modern postwar government”, with the power now all in the hands of European leaders.
“Our friends and allies in Europe will now tell us what conditions we must accept to trade in our largest market,” he said. “This is the moment when the empty phrases and undeliverable promises of the Brexiters will be replaced by the hard reality. They will decide. We will be told. It is what every Conservative prime minister I have worked for was determined to avoid.”
But Iain Duncan Smith, a former Conservative leader and longstanding Brexit campaigner, insisted that it marked the “end of all of the preamble and the beginning of departure”.
He said: “Tomorrow, ironically, is the day the United Kingdom becomes truly united because it has only one position: that we are leaving the EU.”
The former Liberal Democrat leader Nick Clegg said it was the moment that the “utopian wishful thinking from Brexiters” gave way to hard realities, calling on May to “face down the Brexit zealots in her own party and in the Brexit press”.
However, the former Ukip leader Nigel Farage said of the moment that article 50 would be triggered: “After a quarter of a century spent campaigning for this moment, it will be a big happy day.”
Gisela Stuart, the Labour MP who chaired the Vote Leave campaign, called on colleagues to bring an end to the arguments. “This is when we move on,” she said. “David Cameron called a nationwide referendum, which had a massive turnout and a clear majority. Whether people agreed or not, it is done.”
The prime minister signed the letter shortly after 4.30pm in the cabinet room in Downing Street, next to a union flag and beneath a portrait of Britain’s first prime minister, Sir Robert Walpole.
May called the German chancellor, Angel Merkel, the president of the European Council, Donald Tusk, and the president of the European Commission, Jean Claude Juncker, on Tuesday evening to update them ahead of sending the letter.
A Downing Street spokesperson said: “In separate calls, they agreed that a strong EU was in everyone’s interests and that the UK would remain a close and committed ally. They also agreed on the importance of entering into negotiations in a constructive and positive spirit, and of ensuring a smooth and orderly exit process.”
The six-page document will be handed to Tusk by Britain’s EU ambassador, Sir Tim Barrow, after arriving in Brussels on Tuesday night onboard a Eurostar train. It marks the start of a two-year period in which British and EU27 negotiators will lock horns over questions of citizens’ rights, an exit bill, immigration and a future trading relationship.
The first issue to be placed on the negotiating table is likely to be the status of EU citizens living in the UK and British nationals living on the continent, with some suggesting that the prime minister could be minded to set 29 March, 2017 as a cut-off date for when people will have their rights protected.
However, the prime minister is already facing warnings that the European parliament will veto any Brexit deal that prevents EU citizens who move to the UK in the next two years having their rights protected.
A senior Whitehall source told the Guardian that the government had always made clear it wanted to secure a deal on citizens’ rights and the issue would be a “priority” in negotiations – but said any cut-off date would have to be part of those discussions and so had not been decided.
Other early negotiations will be about the divorce bill itself, with the UK likely to pay anything between nothing and €60bn (£52bn). Only when that is resolved, say the remaining EU countries, will they be prepared to embark on the future trading relationship.
Whatever the situation, Britain is expected to leave the EU by the end of March 2019, ending a membership that dates back to January 1973 and was once approved by the public in a referendum.
Downing Street has tightly controlled the impending announcement. Pro-Brexit cabinet ministers are expected to stay out of the limelight, while Tory MPs are attempting not to appear too jubilant.
May knows that she will also have to battle to keep the UK together after Holyrood voted to give Nicola Sturgeon the power to negotiate the terms of a second independence referendum. Warning that Scotland would not be ignored, the SNP’s Westminster leader, Angus Robertson, said that Britain was on track to be “permanently poorer” from the Tories’ Brexit negotiations.
He was one of a number of high-profile remain campaigners piling pressure on to May not to forget the needs of the 48% of the electorate who wanted the UK to stay inside the EU.
Nick Herbert, the Tory MP who chaired his party’s remain campaign, wrote in the Guardian that anyone warning against hard Brexit was branded as “heretics who must recant and swear adherence to the new faith”.
A letter to the Guardian from more than a dozen high-profile figures including the Labour MP Clive Lewis, the co-leader of the Green party Caroline Lucas, and the general secretary of Unison Dave Prentis, claimed the government was pursuing a “harmful, extreme form of Brexit for which it has no democratic mandate”.
Some will hope that Brexit can still be averted if May fails to hammer out a deal and then is defeated in a general election. Some legal experts, and the man who drafted article 50 in the first place – Lord Kerr – have said the process is reversible, although the government has made clear that it believes the “point of no return” for Brexit has passed.
The former cabinet minister and longstanding Eurosceptic John Redwood insisted that he believed it was a “hugely significant moment” – and was now irreversible.
“We either leave by agreement within the next two years or we leave without agreement on 29 March 2019,” he said. “I’m overjoyed. I think the sooner we are free and able to make our own laws and spend our own money the better. I don’t see the harm coming from it all.”
But the pro-EU Conservative MP and Open Britain supporter Anna Soubry made clear that while the “phoney war” was over, there was still a fight to be had. “Britain will begin walking the path of Brexit, and the wishes of those who voted to leave in the referendum will have been honoured,” she said. “But this is the beginning, not the end.
“It is crucial that in this two-year period the voices and concerns of those who want to preserve close links between Britain and Europe are not shouted down and silenced, and that those with power over this process are held to account.”