After weeks of debate and bitter battles in Parliament, the Article 50 Bill has cleared its final hurdle tonight.
Theresa May is now a position to inform the European Union we are leaving.
That means Britain faces the most intense, arduous and complex negotiations since the end of the Second World War.
Both the EU and the Government agree on one thing: this is ‘unprecedented’ territory and neither side knows how the talks will pan out.
Here is your guide to what happens next, the key players and the issues which could make or break the negotiations.
What happens now?
Theresa May will send a letter before the end of March to the European Union giving notification the UK is leaving.
The Prime Minister’s letter is expected to set out the Government’s wish list for the timetable and contents of the Brexit talks.
The timing has yet to be decided. Downing Street has said Mrs May will trigger Article 50 at the end of March.
This suggests she could wait until March 27, after the summit of EU leaders in Italy on March 25 to mark the Treaty of Rome that led to the founding of the European Economic Community 60 years ago.
When will negotiations start?
No talks will begin for several weeks. The EU is not expecting any negotiations until mid April at the earliest.
The European Commission’s Michel Barnier has been appointed the EU’s chief negotiator.
But Mr Barnier needs to take his instructions from the leaders of the remaining 27 EU countries – the council of Europe chaired by Donald Tusk.
The Brexit talks can only start once the EU27 have met. The European Parliament also has to give the go-ahead for the talks.
A provisional date of April 6 was being discussed by the EU27 for the summit but the timing depends on when Article 50 is triggered.
And even then, there could be delays as there are likely to be technical aspects that need ironing out before final approval to commence the talks is given.
Who will lead the talks?
The negotiations will be overseen by Theresa May and led by David Davis, the Secretary for State for Exiting the EU.
Most of the heavy lifting will be done by Oliver Robbins, the permanent secretary at the Department for Exiting the EU, Sir Tim Barrow, our ambassador to Brussels, and his deputy Shan Morgan.
The EU lead negotiator is Barnier, a right-wing French politician who was the country’s former Foreign minister.
He will be assisted by the Belgian Didier Seeuws, a former chief of Herman Van Rompuy, and a team of 30 officials.
How long will the Brexit negotiations take?
Once Theresa May has triggered Article 50 there are two years to complete the negotiations.
But Mr Barnier has said that in reality there will be just 18 months.
No negotiations can begin until the European Council has authorised the guidelines, which could take several weeks.
Then the talks have to be completed in time for the European Parliament and the European Council to ratify them.
This means the talks will have to finished by October 2018.
Will they be done in that time?
Watch in full. Theresa May outlines her Brexit plan
There is an outside chance that an agreement could be struck before October 2018 but most accept it will take much longer.
Mrs May has indicated the UK will seek “transitional arrangements” – this means we could continue trading under EU rules to allow the talks to be extended.
But this would require the approval of the European Parliament and the other 27 countries.
What other problems does Theresa May face?
The final Brexit deal – approving the UK leaving the EU – can be ratified by a qualified majority of the 27 EU leaders.
But any new trade deal requires an unanimous vote of all 27 countries and most likely the approval of their national and, in some countries, regional parliaments.
A free trade between Canada and the EU was almost derailed at the last moment because the Walloon region of Belgium objected to the terms .
The UK could face a similar stumbling block.
What is the first item they will discuss?
The first item on the agenda will be to agree the format and timetable for the negotiations.
Britain believes the wording of Article 50 means we can negotiate a new trade treaty at the same time as negotiating our exit from the EU.
The first flashpoint could be if the European Commission says that we have to agree our exit before any talks on trade can begin.
The negotiations will then cover the divorce bill, rights of EU and UK citizens, access to the single market and the customs union and our membership of various EU bodies such as the European Medicines Agency, Europol and the European Aviation Space Agency.
What are the chances of getting a deal?
Theresa May wants the “greatest possible access” to the single market through a free trade deal. She also wants some form of customs union agreement.
At the same time she wants to regain control of borders and end the UK being subject to the European Court of Justice.
Barnier has said his priority is to “preserve the unity” of the EU27.
He has also said that cherry picking by the UK “is not an option” when it comes to single market access.
The talks will hinge on how much the EU is willing to put its commercial interests ahead of its political ones.
It will want to maintain trading links with the UK but will not want to give special favours that will undermine the cohesion of the EU by granting non members equal conditions to members.
And everything could be thrown up in the air if France elects Marine Le Pen in May and Angela Merkel loses the German elections in September