Theresa May will trigger article 50 on Wednesday 29 March, the prime minister's spokesman has confirmed.
The UK's permanent representative to the European Union, Sir Tim Barrow, notified the EU on Monday morning that a letter should be expected on that date.
May, who was visiting Swansea on Monday, intended to visit Northern Ireland and Scotland before the formal notification was sent by letter on 29 March, Downing Street said.
Downing Street sources had repeatedly said that Theresa May would take the action to start the Brexit process "by the end of March".
Earlier rumours that the step would be taken last week were quashed by sources close to the Prime Minister after Nicola Sturgeon declared her desire for a second independence referendum, leading to speculation that May did not wish to appear cavalier about the future of the union.
Brexit secretary David Davis confirmed that Article 50 would be triggered next week, calling the process "the most important negotiation for this country in a generation."
He went on: "The Government is clear in its aims: a deal that works for every nation and region of the UK and indeed for all of Europe - a new, positive partnership between the UK and our friends and allies in the European Union."
More details soon ...