London says the Brexit process is irreversible once Article 50 is triggered, although experts have said there is no legal ban on member states changing their minds before leaving the EU.
May, 60, hopes to negotiate terms that keep trade, financial and political relations with European Union member states as close as possible after Brexit, but also satisfy eurosceptics in her Conservative Party who demand a complete break from an institution they say has stolen British sovereignty.
Notification of the historic step will come in the form of a letter from the Prime Minister to Mr Tusk, though Downing Street did not make clear whether this would be a physical letter handed to the European Council president by a United Kingdom representative or might be sent electronically.
Britain's ambassador to the EU, Tim Barrow, told the office of European Council President Donald Tusk on Monday morning that Britain would trigger Article 50 of the Lisbon Treaty next Wednesday, May's spokesman said.
"After we trigger, the 27 will agree their guidelines for negotiations and the Commission's negotiating mandate", Slack said.
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".
That sets a clock ticking: Article 50 says that two years from the moment of notification, "the Treaties shall cease to apply" and Britain will no longer be an European Union member.
After months of waiting, it feels like we are in the slightly unusual period just before the business end of Brexit gets underway.
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Russell, the minister for United Kingdom negotiations on Scotland's place in Europe, has been involved in discussions between London and Edinburgh over the Article 50 process as a member of the Joint Ministerial Committee on EU Negotiations.
Speculation remains rife in Westminster that May will use the triggering of Article 50 as the springboard to announce a snap general election in May.
European leaders have stepped up their rhetoric over the past week as the start of negotiations draws nearer.
The spokesperson added that Downing Street expects a response from European Union up to 48 hours afterwards.
Her visit comes after she rejected calls for a potential second independence referendum for Scotland before a deal is reached.
Britain voted 52 percent to 48 percent to leave the European Union in a referendum on June 23.
Autumn 2018 to Spring 2019 - Just to make things complicated, Scottish government wants independence vote once Brexit deal is clear.