Article 50 triggered: When will Brexit happen?

Theresa May's letter signalling the UK's departure from the European Union will be delivered at the office of European Council President Donald Tusk today.

When the "divorce" notice is handed over at 12.30pm, the Brexit countdown begins.

The "'Dear Donald' letter will launch a flurry of diplomatic activity in Brussels", says The Guardian, beginning with Tusk's response.

He is expected to circulate a draft response to EU27 – the remaining member states - on 31 March, providing the first official insight into how the bloc intends to approach the talks.

For the next two years, there will be discussion, debate and most probably disagreements as EU and UK negotiators attempt to reach an agreement on the country's relationship with the bloc.

What are the next steps?

April 2017: The EU-27 nations meet to agree on Brexit guidelines, outlining the broad strokes of their response to Britain's decision to withdraw from the EU.

May-June 2017: Member states will get together behind the scenes to draw up more detailed negotiating instructions to be passed on to the European Commission, which will act as the union's chief negotiator.

At the same time, lawyers in Whitehall will be working flat-out on the great repeal bill for the Queen's Speech in May. This will smooth the legal transition of Brexit and incorporate most existing EU legislation into domestic law, where they can be amended or repealed at a more convenient time.

Christmas 2017: By now, UK and EU negotiators should have come to an agreement on the broader principles of Brexit, including the status of EU nationals living in the UK, and outstanding payments to be made. Detailed discussions of the UK's future relationship with the bloc can now begin.

However, says the New Statesman, these talks are still likely to focus on "a transitional arrangement".

"The real deal that will shape Britain's future outside the EU is the trade deal," it says. "And there's no deadline on that."

October 2018: Brexit could be decided by now. Michel Barnier, the EU Commission's lead negotiator, said last year that "it is clear that the period of actual negotiations will be shorter than two years".

In order for the UK and EU parliaments to ratify a final agreement by March 2019, a deal must be on the table by October 2018, he added.

No one can predict how negotiations will develop over the next 18 months, but leading Leave campaigner Daniel Hannan writes in the Daily Telegraph that despite the sabre-rattling in London and Brussels, "it is in the interests of both sides to reach a deal".

March 2019: If all EU member states and the UK have formally agreed the terms, Brexit can be completed on schedule in March 2019.

What needs to be negotiated?

It has been widely reported that the bloc will demand the UK agrees to pay its £50bn share of EU funds which have already been allocated for future spending before wider talks can begin.

European Commission President Jean-Claude Juncker told the BBC last week that the country will be expected to make a settlement "reflecting former commitments by the British government and by the British parliament", which he confirmed could be in the region of £50bn.

However, "all the figures in the divorce negotiations are likely to be contentious and contested," says the BBC, and the final figure will "almost certainly" be lower.

When more in-depth talks do begin, the status of EU nationals in the UK and British nationals in the EU will be high on the agenda.

The PM has refused to succumb to parliamentary pressure to guarantee EU nationals the right to remain.

But both the UK and the EU have expressed a willingness to come to a favourable arrangement and neither will want to be seen as persecuting people who have made their life in another country.

In addition to all this, negotiators must also address the UK's future trading relationship with the EU, as well as legal jurisdiction, border control and the potential for a transitional agreement.

Infographic by www.statista.com for TheWeek.co.uk