If all goes according to the two year negotiations allowed for in the official timetable, Brexit should happen in March 2019.
May's spokesman said the United Kingdom wanted to start withdrawal negotiations "promptly", but accepts that "it is right that the 27 have a chance to agree their position" before talks start.
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.
"I hope for realism on the sequence of things, realism on the price that it is going to cost, realism on the complexity and thus the timespan that will be necessary, because those are the things that I have missed so far from the British side", he said.
His predecessor, Sir Ivan Rogers - who quit abruptly in January, condemning the Government's "muddled thinking" - warned it would take until the mid-2020s to achieve a free trade deal.
He told the Commons Brexit committee: "We will be having conversations with them beforehand".
He added that Number 10 now wants negotiations to "start promptly" - once the article is invoked it will be up to the European Union to come back, with an early response expected within 48 hours.
"They will all see from the UK's example that leaving the European Union is a bad idea", Juncker said.
The Lib Dem leader argued that the British people did not vote to leave the single market, and so should be given a vote on the final Brexit deal.
May will address MPs in a statement to the House of Commons following her regular weekly session of prime minister's questions on March 29. She will contact Tusk directly to launch negotiations in due course.
European Union leaders will then meet to discuss these, probably in April, and a final negotiating mandate will be agreed, probably by the end of May or early June.
The notification of triggering Article 50 will come in the form of a letter. Brexit secretary David Davis said that there is a Fixed Term Parliament Act, which the prime minister intends to honor.
Scottish leader Nicola Sturgeon threw a wrench in those plans last Monday by announcing a push for a new independence referendum from the United Kingdom, which also includes England, Wales and Northern Ireland.
Hannah White, IfG director of research, said, "The legislation required for Brexit will leave little parliamentary time for anything else - and making a success of it will require a large volume of bills and secondary legislation to be passed by Parliament against a hard deadline".