Article 50 of the EU Treaties sets out a clear two-year deadline for completion of the withdrawal negotiations.
But what happens within that period is far less clear.
Here are some of the milestones expected along the way to Britain's final withdrawal:
March 29: Theresa May will inform the European Council of Britain's intention to leave the EU. Within the following 48 hours, the European Commission is expected to issue "draft negotiation guidelines", which will be sent to the 27 remaining states for consultation.
March 30: A white paper will be produced on the Great Repeal Bill - the legislation that will turn more than 40 years of EU regulations into domestic laws.
April 29: An extraordinary European Council summit of the remaining 27 states will be held to agree a mandate for chief negotiator Michel Barnier and clear the way for talks to begin in earnest in May.
Over this period, the European Parliament will also debate and vote on its "red lines" for any deal.
Negotiations are expected to begin with "talks about talks", with arguments over whether divorce and trade discussions should take place simultaneously, as the UK wishes, or whether consideration of future trade relations should be put off until after arrangements for withdrawal are agreed, as the Commission wants.
Then talks could become bogged down in wrangling over a "divorce bill" of as much as €60bn (£52bn) which the Commission will present to cover spending commitments Britain has already signed up to, as well as its share of the cost of pensions for EU officials.
May 4: Local government elections in England, Wales and Scotland will give voters a first opportunity to pass judgment on Mrs May's handling of Brexit negotiations.
May 7: A new president will be elected in France. Victory for the National Front's Marine le Pen could throw the European side into disarray by raising the prospect that France too will quit the EU.
A win for Emmanuel Macron or Francois Fillon may also affect the EU27's negotiating stance.
Summer: Intensive negotiations are expected to continue through the summer, with early discussions on the status of EU citizens living in the UK and British nationals resident on the continent.
Arrangements may also be thrashed out for a "transitional deal" to cover the period between Brexit and the conclusion of trade talks, if these are not completed within the two-year deadline.
September 24: German federal elections could see Angela Merkel replaced as Chancellor by former European Parliament president and staunch federalist Martin Schulz, who once called for the creation of a "genuine European Government".
Successive "rounds" of talks can be expected to take place through the autumn and winter and into 2018, as teams of negotiators from each side gather around the table for several days at a time before retiring to their capitals to prepare for the next bout.
May: English local government elections.
October: This is the target date Mr Barnier has set for concluding withdrawal negotiations, in order to allow time for them to be ratified before the end of the two-year Article 50 deadline.
Scottish First Minister Nicola Sturgeon has said she wants a second referendum on Scottish independence after the terms of the deal are known and before Brexit takes effect in the spring of 2019.
Winter 2018/19: Once a deal is concluded between the Commission and the UK, it will go back to the member states of the EU.
The European Court of Justice could be asked to rule on whether the deal requires approval by each state.
If so, it could have to be ratified by as many as 38 national and regional parliaments across the European Union, with any of them effectively holding a veto.
Mrs May has promised a parliamentary vote on the withdrawal deal, but is offering MPs only the option to "take it or leave it".
Under her plans, rejection of the deal would mean the UK crashing out of the EU without agreement and being forced to trade under disadvantageous World Trade Organisation tariffs.
The PM has promised that the Westminster vote will take place before the European Parliament debates and votes on the deal, effectively giving MEPs the final say on whether it will go ahead.
March 29: Two years after the invocation of Article 50, the UK ceases to be a member of the EU and is no longer subject to its treaties, whether or not a withdrawal agreement has been reached.
This date can be extended for further negotiations by agreement between all member states. It is not yet clear whether the exit clock can be stopped by the UK withdrawing its Article 50 notification.
If no trade deal has been reached by this point, it is possible that UK-EU relations will continue to be governed for months or years after official withdrawal by a "transitional arrangement".
Even if a trade agreement has been sealed, the Government has made clear that it could be introduced gradually during an "implementation phase" after Brexit.
May: European Parliament elections will take place without the UK.
May 7: Scheduled date for the first UK general election following Brexit.