LONDON, March 29 -- British Prime Minister Theresa May and her most senior ministers gathered in the Cabinet room at 10 Downing Street Wednesday at the start of one of the most historic days in generations for Britain.

The ministers, representing every government department, met to discuss the letter to be personally delivered later Wednesday to European Council President Donald Tusk to signal Britain's departure from the European Union(EU).

In a carefully choreographed piece of political theater, a timetable has been worked out for the day's running order.

Media reports in London said the historic day started in the early hours as a government official, accompanied by security guards, boarded a Eurostar train for the journey to Brussels, with the departure letter in a government briefcase.

May had signed the letter Tuesday night, addressed to Tusk. In Brussels it was handed to Tim Barrow, the British ambassador to the EU.

Barrow will personally hand the letter to Tusk at precisely 1:30 pm local time (1130 GMT) in Brussels. At the same time in Westminster, May will address MPs in the House of Commons where she will confirm to politicians that the deed has been done.

Politicians and political commentators on both sides of the leave and remain sides will scrutinize every word of the letter to look for clues on how Britain plans to proceed with its departure from the EU.

The handing of the letter to Tusk will formally see the commencement of negotiations of Britain's future relationship with the EU.

Britain will remain a full and working members of the EU until a departure point arrives, around March 2019.

The fate of around 3.2 million European citizens currently working or living in Britain, and more than 1 million British nationals living in EU member states, is expected to be high on the early agenda.

Foreign Secretary Boris Johnson, leaving Downing Street after the early morning Cabinet meeting, told waiting journalists: "Today is a great day, a historic day."

May will head to the Houses of Parliament for her weekly session of Prime Minister's Questions at noon. She is expected to be bombarded with questions about domestic issues.

But politicians will eagerly await her statement on her triggering article 50, the process she has to follow under EU rules to enable Britain to withdraw from the bloc.

May advisors have already briefed the media, saying May's speech will call for unity of the country whichever side of the Brexit debate they were on.

May promised more than six months ago to trigger article 50 by the end of March. She faced legal challenges, passionate debates in both houses of the parliament, and came through all these unscathed.

It seems everybody in Britain is gripped by Brexit fever, even the famous Downing Street cat, peering through the railings as an army of world media people gathered outside Number 10 to capture images of an historic day.

Britain joined what was the European Economic Community (EEC), forerunner to the EU on January 1, 1973. Two years later the people of Britain in the first referendum voted to remain as a member.

On June 23, in a second referendum of EU membership, the people of Britain voted to leave.