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 May would take the action to start the Brexit process “by the end of March”.
Earlier rumours that the move would be made last week were quashed by sources close to the prime minister after Nicola Sturgeon declared her desire for a second Scotland independence referendum, leading to speculation that May did not wish to appear cavalier about the future of the union.
The Brexit secretary, David Davis, confirmed article 50 would be triggered next week, calling the process “the most important negotiation for this country in a generation”.
“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.”
The prime minister’s spokesman said Barrow had a conversation with the officer of the European council president, Donald Tusk, earlier on Monday to give the EU notice of the date. “There will be a letter, [May] will notify President Tusk in writing, and the prime minister has already confirmed she will give a statement to parliament as well,” he said. “More details will be given in due course.” (via the Guardian)
Negotiations are expected to last two years.
Meanwhile in Scotland, first Minister Nicola Sturgeon insists on a independence referendum and is sure she will win.
Last week, Sturgeon invited EU nationals to make Scotland their home.