London says the Brexit process is irreversible once Article 50 is triggered, although experts have said there is no legal ban on member states changing their minds before leaving the EU.
Expect the real action to start in late May or early June, when the two sides sit down to start the talks. Suggestions from the European Union side that this bill could reach €60 billion will be strongly resisted by Britain. Such a transitional deal would allay fears about an impending cliff edge in March 2019, when Britain is set to crash out of the single market and face the EU's steep external tariffs on goods ranging from meat to cars.
Should no deal be made then the United Kingdom will revert to trading with the European Union under World Trade Organisation rules.
Speaking to the press in Brussels, the Irish finance minister Michael Noonan said negotiations would not get underway for two months following the Article 50 triggering.
"So Britain will exit the European Union on 29 March 2019", the Downing Street spokesperson said. However, beyond the likelihood that it will express the desire to reach a deal with the European Union, it is unlikely to give too many hints on Britain's negotiating stance.
The notification of Article 50 will take the form of a letter to Tusk, likely outlining Britain's key objectives, followed by a statement by May to MPs in the House of Commons. But May has so far rejected the call for a new Scottish referendum until after Britain leaves the EU.
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.
Downing Street said she would write a letter to the European Council, adding that it hoped negotiations on the terms of exit and future relations could then begin as quickly as possible. "There will need to be a longer time for the U.K.to dismember itself from the EU and a transition deal can include all exit issue plus trade", Fredrik Erixon, director of the European Centre for International Political Economy told CNBC via email.
A transitional deal for about five years is certainly an option.
"The EU's permanent representative to the EU informed the office of Donald Tusk that it's the UK's intention to trigger Article 50 on March 29", a spokesperson for Prime Minister Theresa May said.
The EU leaders will later hold a summit in April or May to decide a final response.
There are vital issues here for Ireland: the future of the Border and the Common Travel Area and the ability of Irish people to work freely in the United Kingdom and vice versa. "We want negotiations to start promptly but it's obviously right that the 27 have an opportunity to agree their position".
The mood music will also be important.
2018 - May wants to negotiate a comprehensive free trade deal.