Theresa May will trigger article 50 on Wednesday 29 March, the prime ministers spokesman has confirmed.
The UKs 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 May would take the action to start the Brexit process by the end of March.