The timing of Article 50 was up to Britain. What happens next is up to the EU.

European Commission spokesman Margaritis Schinas said Monday that the EU is "ready to begin negotiations."

Tusk has said that that once EU officials get Britain's notification, they will respond within 48 hours, offering draft negotiating guidelines for the 27 remaining member states to consider. Leaders of the 27 nations will then meet in April or May to finalize their negotiating platform.

U.K. Brexit Secretary David Davis has said "the first meeting, bluntly, will be about how we do this? How many meetings, you know, who's going to meet, who's going to come."

Substantial talks may have to wait until after France's two-round April-May election for a new president. Another hiccup could be Germany's September election, which will determine whether Chancellor Angela Merkel gets another term.