Theresa May has signed the letter that will trigger Article 50 of the Lisbon Treaty, the formal mechanism for a state wishing to exit the EU.
It will be delivered to the European Council at about 12.30 BST, at which point the prime minister will make a statement to the Commons about its contents and her expectations for the two years of negotiations and ratifications ahead.
As it stands, the UK is on course to leave the EU on 29 March 2019.
In a statement last night, she urged everyone to put the differences of last year's referendum aside and unite to secure the best deal for the UK.
Hello and welcome to our rolling coverage of what is a historic day for the UK - the day when Theresa May officially triggers the two year process of leaving the EU. We'll be bringing you all the action from the UK and continental Europe and further afield as it happens in text and video.