Triggering Article 50 of the EU's Lisbon Treaty, the formal procedure for leaving the bloc, will open a two-year timetable for hard negotiations, meaning Britain could be out of the European Union by 2019.
The EU is "ready to begin negotiations", European Commission spokesman Margaritis Schinas told reporters in Brussels.
Parliament could be tasked with scrutinising up to 15 new bills to deliver Brexit, leaving little time for other unrelated legislation, the Institute for Government has warned.
May has said since October that she intends to trigger Brexit by the end of March, without being more precise.
The European Observer explains that if parliament rejects the deal, the United Kingdom will leave the EU anyway and would fall back to trading with the bloc on the basis of World Trade Organisation (WTO) rules.
"There is no queue to join the European Union and we have had several voices over recent times saying that if Scotland wanted to be in the EU then there would be a very open warm reception for that", she said.
Tusk tweeted his response to the news, confirming that "within 48 hours of the United Kingdom triggering Article 50, I will present the draft #Brexit guidelines to the EU27 Member States".
The trigger for all this tumult is the innocuous-sounding Article 50 of the EU's Lisbon Treaty, a never-before-used mechanism for withdrawing from the bloc.
Ms Sturgeon said she is determined to hold the ballot and said Scotland would seek to remain part of the European Union. Tusk will also summon those 27 leaders for a summit to endorse the final guidelines, probably in early May.
Britain's envoy to Brussels, Sir Tim Barrow, "has informed the European Union this morning that Britain will trigger Article 50 on March 29", the Government said.
He claimed he had not been informed by Downing Street about the Prime Minister's intended date for triggering Article 50.
"The most important negotiation for this country in a generation", said Brexit Secretary David Davis.