BRUSSELS (Reuters) - Prime Minister Theresa May will write to European Council President Donald Tusk this month to trigger Britain’s withdrawal from the European Union under Article 50 of the EU treaty.

Prime Minister Theresa May holds a regional cabinet meeting in Runcorn, Cheshire, as she launched her industrial strategy for post-Brexit Britain with a promise the Government will "step up" and take an active role in backing business, Britain, January 23, 2017. REUTERS/Stefan Rousseau/Pool

It should be out within two years. Here is a timeline:

THE ARTICLE 50 LETTER

March 13 - The earliest the British parliament can complete approvals to launch what Britain says is an irrevocable divorce.

March 15 - Tusk wants May’s letter in mid-March so leaders can react at an EU summit on April 6. Letter should be public.

March 20 - To frustrate Scotland’s ruling, anti-Brexit SNP, May might hold off till after the SNP congress on March 17-18.

March 25 - The other 27 EU leaders meet in Rome to mark 60 years of founding treaty. May wants to avoid spoiling the party by filing for divorce around then. Self-set deadline March 31.

SUMMIT, GUIDELINES, RECOMMENDATIONS

April 6-7 - EU27 have pencilled summit to agree guidelines for EU executive negotiating team led by Michel Barnier. EU needs about four weeks to prepare the summit, so any delay in letter may push it back.

April 14 - Good Friday. If Tusk can’t hold summit before this, Easter holidays across Europe will delay it until after...

April 23 - French presidential election begins but before...

May 15 - French President Francois Hollande hands over to a successor elected in second round of voting on May 7.

April-May - Barnier will reply to leaders, possibly in days, with his detailed “recommendations” of how to structure talks.

DIRECTIVES

A series of ministerial Council meetings, specialized by topic, will be called, again excluding Britain, to agree legal “negotiating directives” that will bind Barnier and his team.

FACE TO FACE

After nine months of phoney war since the June 23 referendum vote to quit, British negotiators led by Brexit Secretary David Davis will finally sit down with EU, possibly 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 Withdrawal Treaty by year’s end, e.g.: 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 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. An idea of customs plans may be needed to resolve eg Irish border problem.

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

B-DAY

March 15, 2019? - Britain will leave the European Union. At any rate, it should leave two years after May’s letter.

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.