A spokesman for the PM said that the United Kingdom has informed European Council president Donald Tusk's office of May's plan to formally trigger Britain's exit from the EU.
David Davis, Britain's Brexit secretary called the departure from the European Union "the most important negotiation for this country in a generation".
Asked if the devolved governments had been informed this notification was being given on Monday, the Prime Minister's official spokesman said: "We are informing you all essentially now". "We are ready to begin negotiations". Notification comes 279 days after the referendum of June 23 a year ago delivered a 52 per cent to 48 per cent majority in favour of withdrawal.
A two-year timetable is expected to be followed, meaning Britain's official break with the European Union should occur in March 2019.
Theresa May will trigger European Union withdrawal talks under Article 50 on March 29, Downing Street has announced.
The split came as Sir Tim gave an update verdict on the prospects for Britain agreeing a framework for future trade with the European Union within the two years of the Article 50 negotiations, to start next month.
Formal talks between Britain and the EU must then wait for member states to approve more detailed negotiating rules and give an official mandate to European Commission Brexit negotiator Michel Barnier, officials said.
'The government is clear in its aims: a deal that works for every nation and region of the United Kingdom and indeed for all of Europe - a new, positive partnership between the United Kingdom and our friends and allies in the European Union'. A spokesperson said that May will write a letter to the EU's 27 other members to express her intent to have the negotiations begin swiftly.
The prime minister will meet First Minister of Wales Carwyn Jones, as well as local businesses, as she tries to show she is including all areas of Britain in negotiations with the EU.
IFG's research director, Dr Hannah White, told BBC Radio Four's Today programme: "The estimates we have heard are something between ten and 15 Bills required in the next two parliamentary sessions". Brexit secretary David Davis said that there is a Fixed Term Parliament Act, which the prime minister intends to honor.
"Theresa May has repeatedly said that she wants to build a national consensus on Brexit, but it is increasingly clear she has failed to do so".
The IFG - an independent charity that aims to increase government effectiveness - says departments will need "ruthlessly to prioritise" other legislation and find non-legislative routes to get the laws through, particularly given the government's narrow Commons majority.