Theresa May has gathered her senior ministers in Downing Street as the UK prepares for a "pivotal moment" in its journey to a future outside the European Union.
The Prime Minister has signed the letter that starts the formal exit process and the historic document will be hand-delivered by a senior diplomat to EU chiefs in Brussels.
Shortly before a Cabinet meeting, Chancellor Philip Hammond said the letter sets the "right tone" and sends the "right signals" to European leaders about how the UK wants to conduct the negotiations that will decide the country's future.
At some time after 12.30pm the premier will inform MPs that Brexit is being triggered and, in Brussels, British ambassador to the EU Sir Tim Barrow will deliver the document to European Council president Donald Tusk.
Once it has been accepted, Article 50 has been officially launched, starting a two-year countdown to the UK leaving the EU.
Mr Hammond said it was an "exciting time", telling the BBC Radio 4 Today programme: "This is a pivotal moment for Britain."
Sir Tim arrived at the European Council clutching a briefcase containing the Article 50 letter as cameras captured the historic moment.
He said "Morning" to reporters as he walked into the Europa Building in the heart of the European quarter, where he is expected to hand the document to Mr Tusk this afternoon.
Mr Hammond said the letter builds on the position the Prime Minister has already set out, indicating that the UK would leave the single market and would not remain a "full member" of the customs union.
"Of course it will go further in expressing how we want to take the negotiation forward and how we see this negotiation developing," he said.
The Chancellor said the Government's position on leaving the single market and customs union would have "consequences" but demonstrated that "we understand that we can't cherry-pick, we can't have our cake and eat it".
He said: "By deciding to leave the EU and negotiate a future relationship with the EU as an independent nation, there will be certain consequences to that and we accept those."
The Chancellor said that although preparations were being made for the possibility of leaving the EU without a deal, he was "absolutely confident" that an agreement on the ongoing relationship would be sealed.
He played down warnings that Brexit would lead to lorries queuing up at Dover as customs checks and red tape meant trade ground to halt.
"It is not in the interests of anybody on the continent of Europe to have lines of trucks," he said.
"I'm very confident that we will not get an outcome that is a worst-case outcome for everybody. That would be ridiculous."
Mr Hammond also indicated that the UK was not seeking to use the triggering of Article 50 as a cut-off date for EU citizens in the UK hoping to see their rights continue after Brexit.
"Of course they can come here after today," he said. "We remain full members of the EU with all the obligations and all the rights of membership."
He also responded to suggestions from Brussels that the EU would seek an exit fee in excess of £50 billion to cover the cost of obligations the UK made while a member.
"I should be clear that we simply do not recognise some of the very large numbers that have been bandied about in Brussels," he said.
The Prime Minister will promise to represent "every person in the UK", including EU nationals, when she takes to the negotiating table.
She will tell the Commons that "as we face the opportunities ahead of us on this momentous journey, our shared values, interests and ambitions can, and must, bring us together".
Key EU figures agreed to enter into Brexit talks in a "positive spirit" during a series of telephone calls with Mrs May on Tuesday evening.
German Chancellor Angela Merkel, Mr Tusk and European Commission president Jean-Claude Juncker said the UK would remain a "close and committed ally".
Within 48 hours, the European Commission is expected to issue "draft negotiation guidelines", which will be sent to the 27 remaining states for consultation.
Their leaders will meet on April 29 at an extraordinary European Council summit to agree a mandate for chief negotiator Michel Barnier and clear the way for talks to begin in earnest in May.
Labour leader Jeremy Corbyn said: "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.
"That's why Labour has set the clear priorities of full access to the European market, rights at work and environmental protection. And we will hold the Government to account every step of the way."
Cabinet members emerged from Number 10 after an hour-long meeting and several, including Foreign Secretary Boris Johnson, smiled as they acknowledged questions shouted from the press pack stationed in Downing Street.
Most simply answered: "Morning."
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