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. British Prime Minister Theresa May, under the Article, will notify Tusk of her nation's intentions to leave the 28-nation bloc.
The article stipulates that the two sides will have until March 2019 to agree on a divorce settlement and — if possible — establish a new relationship between Britain, the world's No. 5 economy, and the EU, a vast single market containing 500 million people.
The European Commission — the bloc's legislative arm — said it stood ready to help launch the negotiations.
"Everything is ready on this side," commission spokesman Margaritis Schinas said. Leaders of the 27 other EU nations will meet by the month of May to finalize their negotiating guidelines.
May's 10 Downing Street office said the prime minister will make a statement in the House of Commons on the day Article 50 is triggered.
Britons voted in a June referendum to leave the EU after more than 40 years of membership. But May was not able to trigger the talks until last week, when the British Parliament approved a bill authorizing the start of Brexit negotiations.
But like any divorce, things may not go to plan.
The letter May sends next week will plunge Britain into a period of intense uncertainty. The country doesn't know what its future relationship with the bloc will look like — whether its businesses will freely be able to trade with the rest of Europe, its students can study abroad or its pensioners will be allowed to retire easily in other EU states. Those things have become part of life in the U.K. since it joined what was then called the European Economic Community in 1973.
It's also not clear what rights the estimated 3 million EU citizens already working and living in Britain will retain.
And it's not even certain that the United Kingdom — made up of England, Scotland, Wales and Northern Ireland — will survive the EU exit intact.
Scotland's nationalist first minister, Nicola Sturgeon, is seeking a referendum on independence within two years. In the same Brexit vote in which most Britons chose to leave the EU, Scottish voters mostly wanted to stay. Sturgeon says Scotland mustn't be "taken down a path that we do not want to go down without a choice."
May has rejected that suggestion, saying "now is not the time" for another referendum on Scottish independence.
Pro-EU Labour Party Lawmaker Pat McFadden said Monday it is now up to May to deliver the good deal for Britain that she has promised.
"The phony period is nearly over, and the real work of negotiations are about to begin," McFadden said.
Conflicts are likely to arise soon. The EU wants Britain to pay a hefty divorce bill — estimates have ranged up to 60 billion euros ($64 billion) — to cover pension liabilities for EU staff and other commitments the U.K. has agreed to.