Theresa May has insisted that Article 50 of the Lisbon treaty will be triggered “no later that the end of March."
This is the formal mechanism which will begin the two year negotiation for the terms of Britain’s exit from the European Union.
It means that the UK will quit the EU by March 31 2019 at the latest, ahead of the European Parliament elections in May of that year.
How is Article 50 is triggered?
The actual triggering of Article 50 is likely to involve Mrs May writing a letter to Donald Tusk, the president of the European Council, setting out Britain’s negotiation demands.
It could take the form of a one line email, or a hand-delivered letter to the European Council in Brussels.
The official declaration could be delivered by David Davis, the Brexit Secretary, or Britain's Ambassador to the EU, Sir Tim Barrow.
Mr Tusk has said that EU leaders will respond to the British government within 48 hours.
He told a press conference in Brussels:
When the U.K. notifies, it is our goal to react with the draft negotiation guidelines for the 27 Member States to consider.
For this I think we need more or less 48 hours.
The Prime Minister will be expected to make a statement in the House of Commons, informing the country that the notification has been given to trigger Brexit.
What will happen after Article 50 is triggered?
The leaders of the other 27 EU member states will hold an emergency meeting, most probably in April to agree a common response to Britain’s demands.
Mr Tusk will then reply from the council to Mrs May after about six weeks, making clear the EU’s negotiating position, formally sounding the starting gun on talks.
With both sides having set out their demands, talks will begin between British officials and bureaucrats from the European Commission about the terms of Britain’s exit.