(Updates timing of Commission recommendations)

By Alastair Macdonald

BRUSSELS, March 20 (Reuters) - Prime Minister Theresa May will trigger Britain's withdrawal from the European Union under Article 50 of the EU treaty on March 29, her spokesman said on Monday.

The country should be out in two years. Here is a timeline:

THE ARTICLE 50 LETTER

Wednesday, March 29 - May will write to European Council President Donald Tusk to trigger the withdrawal process.

SUMMIT, GUIDELINES, RECOMMENDATIONS

March 29-31 - Within 48 hours, Tusk sends to the 27 other member states his draft negotiating guidelines.

Saturday, April 29 - The other 27 EU leaders meet to agree final guidelines and mandate the EU executive's Michel Barnier to negotiate.

April 29-May 2 - Barnier will "immediately" reply to leaders with his detailed recommendations of how to structure talks.

Monday May 1 is a public holiday in Brussels and much of Europe.

DIRECTIVES

May - Ministers in the EU's normally low-key General Affairs Council (GAC) will be called, again excluding Britain, to agree legal "negotiating directives" that will bind Barnier. The GAC has a routine meeting scheduled on May 16 but could meet at any time.

FACE TO FACE

Finally, after nearly a year of phoney war since the June 23 referendum vote to quit, British negotiators led by Brexit Secretary David Davis will sit down with the EU, possibly still in May. Talks may start with what to discuss first and how to split up topics.

THE DIVORCE DEAL

December 2017 - Brussels wants a basic deal on a Withdrawal Treaty by year's end. Key issues: the exit bill for Britain's outstanding commitments; treatment of British and EU expats; dealing with outstanding EU legal cases; new border rules.

TRANSITION TO FUTURE RELATIONSHIP

2018 - May wants to negotiate a comprehensive free trade deal. Few see two years as enough time to agree one and Brussels wants to hold off starting talks until after a divorce deal. But London and some EU states may push for parallel trade talks.

October 2018 - Barnier's target to finalise the Withdrawal Treaty, to give time for ratification by the European Parliament and a majority in the European Council by March 2019.

Autumn 2018 to Spring 2019 - Just to make things complicated, the Scottish government wants an independence vote once a Brexit deal is clear. But, May has so far rejected the call for a new Scottish referendum until after Britain leaves the EU.

B-DAY

Friday, March 29, 2019 (tentative) - Britain leaves. At any rate, it should do so two years after May sends the Article 50 letter. It is, as it happens, the last working day of the first quarter of the year.

The date could be fine-tuned. Britain could leave earlier if it gets a deal, and the two-year deadline can be extended if all agree. But Brussels wants Britain out before EU elections in May 2019. Despite mutual threats of no deal, few want such chaos.

A PERIOD OF TRANSITION

May and EU leaders say transitional arrangements may well be needed, to give more time to agree a future trade deal and give people and businesses time to adjust to the divorce. Many see another two to five years after Brexit for a final settlement.

If Scotland votes for independence, expect more years to negotiate its split from London and possible re-entry to the EU. (Editing by Alison Williams)

© Reuters News 2017