Here are the key milestones expected as Britain leaves the European Union:
January 31 - The EU (Withdrawal) Bill is expected to clear its second reading in the House of Lords.
February 21 - The Brexit Bill enters the committee stage in the Lords, likely to run on Mondays and Wednesdays for 10 days.
March 22-23 - A European Council summit in Brussels is expected to reach an agreement on an implementation period, clearing the way for exploratory talks on trade.
April - The Brexit Bill will be discussed for five days at report stage in the Lords.
May 3 - English local government elections will provide the Prime Minister with her first widespread electoral test since the disastrous snap election of June 8 2017.
May - The Brexit Bill is expected to be bounced between the Lords and the Commons in a bout of Parliamentary ping pong after the elections.
October 18 - The legally-binding treaty setting out the terms of the UK's withdrawal agreement and transition period is expected to be concluded. Alongside this, EU chief Brexit negotiator Michel Barnier hopes to be able to publish a political document setting out an agreement on the shape of a future treaty - or treaties - on trade and other issues like aviation, security and judicial co-operation.
Winter/spring - Ratification of the withdrawal and transition treaty is expected, with MPs in Westminster being given a vote on whether or not to accept the deal, followed by a vote in the European Parliament, which holds a veto.
March - Final European Council summit at which the UK will take part as a member.
March 29 - Two years after the invocation of Article 50, the UK ceases to be a member of the EU. Because the exact moment of exit is midnight Brussels time, the UK is due to leave at 11pm on March 29. Under the terms of the EU (Withdrawal) Bill, the bulk of Brussels legislation would be automatically transposed on to the UK statute book. Under the European Commission proposals, Britain would continue to observe EU rules and regulations while having no say in them.
June - European Parliament elections will take place without the UK.
Mr Barnier envisages negotiations on a trade deal taking place during a 21-month transition period in 2019 and 2020 while Theresa May believes the deal can be concluded by March 2019 allowing the period to be used for the implementation of changes.
The UK will seek talks with other countries on free trade deals, though the Commission insists that these cannot be signed until the transition period is over.
Intensive work can be expected on practical arrangements such as the establishment of new regulatory agencies, recruitment of customs and immigration officers and amendment of business contracts.
December 31 - The European Commission's proposed date for the end of the transition period, coinciding with the end of the EU's seven-year budget, which includes UK contributions. Brexit Secretary David Davis has said it could be a few months either side.