Prime Minister David Cameron is to meet European leaders for an EU Council summit for the first time since the UK voted to leave the 28-member bloc.
The meeting will mostly be devoted to a discussion on the political consequences of the UK referendum.
The Prime Minister has not been invited to the second day of the summit, but tonight there will be a dinner, where Cameron will explain the situation in the UK after the referendum, followed by a first exchange of views.
On Wednesday, the other 27 leaders will talk about the "divorce process" as provided by Article 50.
Meanwhile, European Council President Donald Tusk will "launch a wider reflection on the future of the EU".
Baroness Sharon Bowles, former Liberal Democrat MEP and experienced EU negotiator, told the BBC that she expects the focus from EU leaders today to be on "trying to stop what they call contagion". That is, they will want to put an end to the possibility of other nations following the UK out of the door - and they'll most likely announce measures to that end.
And MEPs will also be getting together. At 9am UK time there'll be an Extraordinary European Parliament meeting including speeches by Tusk, European Commission President Jean-Claude Juncker and party leaders.
Its expected many will push for the UK to trigger Article 50 immediately.
However German Chancellor Angela Merkel has said she is in no rush, but has added if it was left too long contagion could ensue. French President Francois Hollande and Italian Prime Minister Matteo Renzi backed her up.
Leaders will also address tackling the migratory crisis, deepening of the single market to boost growth and jobs, and enhancing the security of the EU by working more closely with NATO.
The UK will still remain a member of NATO.