British Prime Minister Theresa May triggered Article 50 of the EUs Lisbon Treaty on Wednesday, starting a difficult two-year Brexit negotiation process.

Actual talks between London and Brussels are not expected for several weeks while the European Union formalises its position.

Here are the key steps ahead:

Friday: EU guidelines

European Council President Donald Tusk will issue draft "negotiating guidelines" on Friday. He is due to give a press conference with Maltese Prime Minister Joseph Muscat on Friday in Valletta.

These are overall political red lines for the next two years and will be circulated to the capitals of the 27 remaining EU countries.

Diplomats known as sherpas make further preparations and then ministers will finalise the guidelines in late April.

April 29: summit

EU 27 leaders will hold a special Brexit summit in Brussels, without Britain, to rubber stamp the negotiating guidelines.

End-April: Commission recommendation

A day or two later, the European Commission negotiator Michel Barnier will issue an initial "Recommendation to Open Negotiations" with his suggestions for how the talks should go.

May: Negotiating Directives

The EU 27's European affairs ministers -- the so-called General Affairs Council -- will meet in May to draw up detailed "Negotiating Directives" that will bind Barnier during the talks.

The ministers already have a scheduled meeting on May 16 but could meet sooner for Brexit.

The guidelines will include the three "divorce" issues the EU wants to deal with before talks on any future trade deal: Britain's exit bill, the rights of EU citizens in Britain and vice versa; and the flashpoint border in Northern Ireland.

Mid- to late-May: Talks start

EU ministers will formally give Barnier the mandate to start negotiations so that formal Britain-EU talks actually begin, nearly a year after Britain voted to leave.

Informal talks however could begin earlier to work out practical issues such as what language the talks will be held in -- Barnier is French -- and the timetable

"There's nothing to stop us talking about procedure before we get the mandate as long as we are not actually negotiating," one diplomat told AFP.

2017-2018: future relationship?

The EU says it will only discuss the leaving bill, citizens rights and Northern Ireland at first. It will only move on to a trade deal once they are sorted out.

October 2018: draft deal?

Barnier has set October 2018 as the latest feasible date for a draft Brexit deal to give it time to be approved by the British parliament, by EU leaders and by the European Parliament, which will have the final say.

End 2018-early 2019

The European Parliament will hold a crucial binding vote on the Brexit deal. National parliaments may also vote on it.

2019: Brexit

Britain will formally leave the EU two years after the notification of Article 50.

Whether that happens with a new trade deal included, a transitional arrangement while one is sorted out, or no trade deal remains to be seen.

A decade of trade talks?

Even if Britain does manage to make a deal with the EU, the accord is likely to be partial or transitional.

A full deal for the future relationship will probably take years -- up to seven years according to Tusk, or even a decade, according to reported comments by Britain's former ambassador to the EU.