BRUSSELS, April 29 (Reuters) - European Union leaders agreed a set of negotiating guidelines for talks on Britain's withdrawal from the bloc, which should be completed in two years. Here is a timeline to Brexit, based on a mixture of public information and estimations by EU sources: THE TRIGGER Wednesday, March 29 - British Prime Minister Theresa May sent a letter to European Council President Donald Tusk confirming the plan to leave and formally launching the process, nine months after Britons voted in a referendum on June 23. SUMMIT, GUIDELINES, RECOMMENDATIONS Saturday, April 29 - EU27 leaders met in Brussels to approve 8 pages of negotiating guidelines and mandate Frenchman Michel Barnier, a former European Commissioner, as chief negotiator. Wednesday, May 3 - European Commission, the EU executive, to approve its recommendations to the Council, the body grouping the member states, on how negotiations should be structured. Barnier has worked these out in coordination with the Council and they go into somewhat more detail on what the EU wants. DIRECTIVES Monday, May 22 - EU27 ministers meet to agree negotiating directives. Based on the Commission recommendations, these are the legal texts that give Barnier power to open negotiations and limit him on what he can offer London - for example, he will not be able to discuss a post-Brexit trade deal for now. UK ELECTION Thursday, June 8 - On April 18, May called a snap election she hopes will increase her Conservative Party's slim majority. Some EU officials think that could give her more leeway to compromise, depending on the strength of Brexit hardliners in her party. FACE TO FACE From June 12 - Finally, nearly a year after the referendum, British negotiators, probably led by current Brexit Secretary David Davis, will sit down with Barnier. However, it is possible some "talks about talks" may get underway after May 22. Both sides can save time by fixing procedural arrangements - who will meet whom where, speaking what language and so on. THE DIVORCE DEAL Autumn 2017 - Tusk says the EU27 will review in the autumn whether "sufficient progress" has been made in outlining a Withdrawal Treaty for negotiators to move on to a second phase of talks to discuss a post-Brexit free trade deal. First the EU wants to settle key issues: how to calculate an exit bill for Britain's outstanding commitments; treatment of British and EU expats; dealing with outstanding EU legal cases; new borders. Leaders could discuss progress at a summit on Oct. 19-20 and take a decision on launching Phase 2 at the next, on Dec. 14-15. TIGHT DEADLINE Oct-Nov 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 - The Scottish government wants an independence vote once the terms of a Brexit deal are clear. But May has rejected the call and Scottish voters seem lukewarm. B-DAY Friday, March 29, 2019 - Barring surprises, Britain leaves at midnight, at the end of the last business day of the quarter. A PERIOD OF TRANSITION May and EU leaders say transitional arrangements may well be needed, to allow 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.