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 Barrow 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.

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."

The Prime Minister will promise to represent "every person in the UK", including EU nationals, when she takes to the negotiating table.

More to follow.