Then comes the hard part - the arguments, the lawyers, the squabbles over money.
Amid fears of the impact on jobs and growth of leaving the single market, May is pushing for "maximum possible access" for British companies.
The Prime Minister will give a statement to MPs in the House of Commons next week, after formally pulling the Brexit trigger.
Also on Monday, the IfG published research warning that up to 15 new bills in addition to the Great Repeal Bill could be required to deliver Brexit.
If all goes according to the two year negotiations set out in the official timetable, Brexit should happen in March 2019.
Scotland's First Minister Nicola Sturgeon wants her country to hold a referendum on Scottish independence immediately after Britain's exit from the EU.
The spokesman said that Britain's Ambassador to the EU Tim Barrow informed the European Council earlier on Monday that Article 50 would be triggered next Wednesday.
David Davis, the UK Government's Brexit Secretary, hinted last week that the devolved administrations would get notice of the triggering of Article 50.
President of the US, Donald Trump, hit Twitter hours after meeting with the German Chancellor Angela Merkel demanding that Germany pay more to the United States for its defence.
On the British side, Davis will take the lead, reporting to May.
The European Commission said it stood ready to help launch the negotiations. A Commission spokesman said on Monday Barnier would do this "immediately" after the summit. He'll receive direction from the Council, which represents the leaders of the member states. He confirms that he will campaign for Britain to remain in the 28-nation bloc.
Of the United Kingdom's four nations, Scotland and Northern Ireland voted to stay in the European Union in the June 23 vote while England and Wales voted to leave.
The first major battle is likely to be about money.
Meanwhile, an European Union spokesperson told reporters in Brussels that they are "ready and waiting" for the letter.
British negotiators are sure to quibble over the size of that tab.
Without a new trade agreement, Britain would fall back on World Trade Organization (WTO) rules, which could mean higher export tariffs and other barriers.
The Prime Minister is attempting to reach out in order to address criticisms that she is not paying proper attention to different administrations.
The government has indicated that Britain could make contributions to the European Union budget to ensure trade access.
The letter May sends next week will plunge Britain into a period of intense uncertainty. "There is not going to be an early general election". But it's unclear whether Article 50 is legally reversible.
As per May's Ocotber speech on the subject, the letter formally submitting parliament's intention to start the Brexit process was expected to reach the European Union by the end of March.