Factbox: Five questions for Britain and Europe after Brexit vote

By Alastair Macdonald BRUSSELS (Reuters) - Following are answers to key questions on what will happen next in Britain's relations with the European Union following its referendum vote to leave: 1. WHAT DOES IT ALL MEAN? The EU is in shock and entering uncharted territory. No member state has ever left, and Article 50 of the EU treaty, which sets out how a state can exit the bloc, offers little detail. Although it provides a sketchy legal framework for a two-year period of withdrawal (see below), many believe it will take longer to establish a new trading relationship between Britain and the EU. Some fear the process will become bitter, disrupting the economy and European affairs across the board. There is already a tug-of-war over Britain delivering the formal notice to quit that will set the two-year clock ticking. Prime Minister David Cameron, who said before the vote that Article 50 would be triggered straight away, announced on Friday he would resign and leave that task to a successor who may not be chosen by the Conservative party until October. One potential successor, Boris Johnson, sees no rush. Some Brexit campaigners argue for negotiating a comprehensive new relationship with the EU, including access for British business to the EU market, before triggering the mechanism, which states Britain must leave with no deal at all if none is agreed within two years. EU leaders demand Cameron start the clock immediately but are divided on whether that means days or, maybe, a few months. In any case, they see no way to force his hand, beyond appeals to his "honor" and that he respect the will of his own voters. A deal Cameron struck with EU leaders in February to curb EU immigration, protect London finance interests from the euro zone and opt out of "ever closer union" has been killed by the referendum result, and EU leaders have ruled out new talks on a different form of British EU membership - "Leave means leave". The stalemate essentially results from dilemmas on either side: Whoever speaks for Britain wants to be seen to honor the referendum result and also get the best possible EU trade deal, which it is unlikely can be negotiated in just two years; the EU is anxious for minimum economic disruption, which implies a fast move to a new deal with Britain but fears conceding anything new to London will trigger a domino effect wrecking the whole Union. Many EU officials see two treaties with Britain being worked on in parallel: a "divorce" pact under Article 50, unwinding the EU budget for example, which would have to be agreed by a weighted majority of the 27 others and done in under two years; and a separate deal on a new economic relationship that would require unanimous acceptance by the bloc and which most expect would take longer to complete. That mechanistic approach may not give the kind of smooth transition leaders like German Chancellor Angela Merkel say they want and Brussels' creative officials and diplomats are certain to consider a range of flexible options. The two-year limit is intended to shorten uncertainty but can be dodged: the leave notice could be held back; a unanimous decision by all 28 states can extend it (though many question how feasible that unanimity is); and one EU official has said a departure treaty negotiated within two years, and approved by a majority, might carry a delayed action clause to bring it into effect only later, once a broader new relationship was agreed. There are a number of options open to Britain, including to maintain access to EU markets as Switzerland and Norway have -- although EU leaders have said the price for that could be allowing free EU migration and accepting other EU rules that British voters have just rejected in the referendum. For now, Britain remains, in principle, a full member of the EU but will be excluded from discussions affecting its exit terms. In practice, many expect British ministers and lawmakers to be rapidly frozen out of much of the Union's affairs. Some Brexit campaigners have also said Britain should act more quickly, for example to stop funding the EU budget or curb immigration from EU states. That could provoke EU reprisals. An array of laws and EU entitlements will cease to apply to British business and citizens, creating what Brexit campaigners say will be opportunities for more growth and more selective immigration but which Cameron has said will do long-term damage to the economy and Britain's global influence. 2. WHAT'S HAPPENING RIGHT NOW? EU leaders and the heads of EU institutions in Brussels have delivered statements that broadly stress a mantra of Three Rs: Regret - at losing nearly a fifth of the EU economy and more of its military and global clout; Respect - for the will of the British people; and Resolve - to keep the other 27 together. They also reminded Britain that it remains a full member for the time being, with all the rights and obligations that entails. EU "sherpa" advisers to the leaders meet in Brussels at 2:30 p.m. (1230 GMT) on Sunday, when a Spanish general election will also affect EU business. On Monday, EU summit chair Donald Tusk and French President Francois Hollande meet in Paris at 11 a.m. and then travel to Berlin to meet German Chancellor Angela Merkel and Italian Prime Minister Matteo Renzi at 3 p.m. Jean-Claude Juncker, who leads the EU's executive Commission which will negotiate the details of a deal with Britain, plans a meeting of its college of national commissioners for Monday. Britain's commissioner, Cameron ally Jonathan Hill, resigned on Saturday. It was untenable to keep a key portfolio overseeing financial services, a battleground of the Brexit negotiations. Cameron has left it to his own successor to decide how to use the Commission seat Britain is entitled to until it leaves. EU leaders meet in Brussels for a 24-hour summit starting at 5 p.m. on Tuesday. EU officials expect Cameron to report on the vote and what Britain will do next over dinner, then go home. Tusk will chair a meeting on Wednesday of the remaining 27, a format that will become familiar in the coming divorce talks. Leaders may agree to meet again in July. 3. WHAT IS ARTICLE 50? These are the key phrases of its 261 words: - A Member State which decides to withdraw shall notify the European Council of its intention ... The Union shall negotiate and conclude an agreement with that State, setting out the arrangements for its withdrawal, taking account of the framework for its future relationship with the Union. - It shall be concluded ... by the Council, acting by a qualified majority. - The Treaties shall cease to apply to the State in question from the date of entry into force of the withdrawal agreement or, failing that, two years after the notification ... unless the European Council, in agreement with the Member State concerned, unanimously decides to extend this period. - The member of the European Council or of the Council representing the withdrawing Member State shall not participate in the discussions ... or in decisions concerning it. 4. WHERE DOES THE EU GO FROM HERE? The Union needs quickly to fill a 7 billion euro hole in its 145 billion euro annual budget, which is currently fixed out to 2020, as it loses Britain's contributions while saving on what Britons receive from EU accounts. The EU will also want to clarify as quickly as possible the status of firms and individuals currently using their EU rights to trade, work and live on either side of a new UK-EU frontier. Britain is expected to give up its six-month presidency of EU ministerial councils, due to start in July next year. Its place may be filled by Estonia or, possibly, Malta or Croatia. EU leaders may push for a quick show of unity on holding the bloc together in the face of eurosceptics inspired by the result in Britain -- including National Front leader Marine Le Pen, who leads polls for next April's French presidential election. But there is little prospect of major new projects. Divisions between Berlin and Paris on managing the euro zone probably rule out a big move on that front before both hold elections in 2017. A major EU security and foreign policy review is already on the summit agenda as is a new push to tighten control on irregular immigration from Africa. Many leaders caution against alienating voters by moving too fast on integration, which they say has alienated voters. Summit chair Tusk wants to launch a formal process of reflection on where the Union has failed to connect with people. 5. SO WHAT CHANGES? In principle, nothing changes immediately. Britons remain EU citizens and business continues as before. In practice, many believe trade, investment and political decisions will quickly anticipate British departure from the bloc. The EU could also face a Britain breaking apart as europhile Scots plan another push for independence and seek to join the EU on their own. There is a "Brussels consensus" that Britain must be made an example of for leaving to discourage others and will face a chilly future, cast out to perhaps talk its way back later into some kind of trade access in return for concessions such as free migration from inside the bloc and contributions to the EU budget - things which Brexit voters want to end but which the likes of Norway and Switzerland have accepted in varying forms. However, cautious diplomats do not rule out surprise turns. There is even speculation that Britons could try to step back from the brink -- the referendum itself is not binding. EU leaders would not -- indeed could not -- throw Britain out if a London government simply decided to ignore the Leave vote. In theory, if London holds fire on Article 50 but pushes for a new membership deal, the EU would have to accept its remaining an awkward presence at the table. Patience for a soured marriage is already short, however, and other states would probably look for ways to end strife that would risk destabilizing the Union. (Editing by Mark John, Janet McBride, Anna Willard)