The British government has said it will trigger Article 50, formally starting the process of leaving the European Union, on March 29.
Sir Tim Barrow, the UK's chief permanent representative to the EU, has informed European Council President Donald Tusk's office of the move.
Keir Starmer MP, Labour's Shadow Brexit Secretary, said, "Britain is about to embark on the most complex and important negotiations since World War II, so this a hugely significant moment for the whole country".
Britain leaving the European Union in March 2019 will see the end of a 46 year old membership.
Former British ambassador to the EU John Kerr, who wrote Article 50, says "it is not irrevocable".
Holyrood's Brexit minister has criticised the UK Government after claiming Tory politicians in London "forgot" to inform Scotland of the date when Article 50 will be triggered. Britain had said it hoped the trade deal could be agreed, at least in outline, during the two-year period when the exit talks were being finalised.
If Scotland votes for independence, expect more years to negotiate its split from London and possible re-entry to the EU.
Tusk reiterated on Monday that he would send governments a draft of Brexit negotiating guidelines within 48 hours of May's letter, which will set out Britain's demands for the talks.
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In the referendum held on June 23 previous year, 52 percent of Britons voted to leave the European Union in a historic and surprise move. "And it is good to know that Prime Minister Theresa May shares this view".
Northern Ireland's largest Irish nationalist party Sinn Fein also said that he wants a referendum on splitting from the United Kingdom "as soon as possible" to join the Republic of Ireland.
May has said that Britain's House of Commons and Lords will have a vote on the deal she negotiates but she has insisted the United Kingdom will leave anyway even if Parliament rejects that deal.
That's the legislation that will begin the country's withdrawal from the EU.
"Brexit will place a huge burden on both Parliament and Government departments", the report warns.
"They will all see from the UK's example that leaving the European Union is a bad idea", Juncker told German newspaper Bild am Sonntag.
The referendum exposed geographical and social divisions in Britain that have deepened over arguments about its future relationship with the EU.