Britain may have given advance notice of triggering article 50 in part to avoid any sudden upset to market sentiment.
David Davis, the UK Brexit Secretary, is readying for what "may be the most complicated negotiation of all time".
Britain said Monday it will begin the process of leaving the European Union on March 29, setting an historic and uncharted course to become the first country to withdraw from the bloc by March 2019. "We are going to be out there, negotiating hard, delivering on what the British people voted for". This means that United Kingdom and EU officials will have until the 29th March of 2019 to agree on how the United Kingdom will leave the Union and how they will trade in the future.
Jean Claude Juncker, as European Commission president, will oversee the process, while Barnier and his taskforce of negotiators deal with the United Kingdom government for the next two years.
The prime minister's spokesman declined to give further details of the content of the letter, but senior government officials familiar with the government's thinking expect it to set out a "positive" vision of the future relationship Britain wants with Brussels after Brexit, including a comprehensive free trade agreement. If Britain leaves without a transitional deal, it could lead to the immediate imposition of tariffs and other trade barriers.
The EU27 are then expected to stage an extraordinary summit within four to six weeks to agree a mandate for European Commission chief negotiator Michel Barnier, with talks probably beginning in earnest in May or June.
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Prime Minister Theresa May had already pre-announced the action back in October.
"I believe that the Government's unrealistic expectation of having trade negotiations concluded within two years of triggering Article 50 - with no plan for extending this period - is compounding uncertainty and risks, causing unnecessary damage to our economy", he said.
"No EU law scholar has suggested that with the agreement of the other 27 member states you can not allow a member state to withdraw its notice".
"We are confident we can deliver the necessary reforms within the legislative timetable, and have also been clear that we envisage a phased process of implementation".
Downing Street said: "We want negotiations to start promptly, but it's obviously right the 27 have an opportunity to agree their position".
Lawyers working for the campaign group The People's Challenge have argued that it will legally be possible for the UK Parliament to revoke Article 50 if the choice is between a bad deal and no deal at all. "This is our best chance to sober up the Brits", said one senior European Union diplomat from an anglophile northern state.
Commission President Jean-Claude Juncker warned that London may have to abandon hopes of a trade deal unless it accepts the terms offered by the European Union, which are widely expected to include a hefty divorce bill.