European Council President Donald Tusk arrives at the Government office ahead of the upcoming European Union summit- the first one since Britain voted to quit- in Bratislava, Slovakia, September 15, 2016. REUTERS/Yves Herman

European Parliament President Martin Schulz (L) is welcomed by Slovakia's Prime Minister Robert Fico at the Government office ahead of the upcoming European Union summit- the first one since Britain voted to quit- in Bratislava, Slovakia, September 15, 2016. REUTERS/Yves Herman

EU leaders attend the European Union summit- the first one since Britain voted to quit- in Bratislava, Slovakia, September 16, 2016. REUTERS/Yves Herman

(L-R) Ireland's Prime Minister Enda Kenny, Cyprus' President Nicos Anastasiades , Denmark's Prime Minister Lars Lokke Rasmussen, Germany's Chancellor Angela Merkel, Belgian Prime Minister Charles Michel and Italy?s Prime Minister Matteo Renzi attend the European Union summit- the first one since Britain voted to quit- in Bratislava, Slovakia, September 16, 2016. REUTERS/Yves Herman

European Union leaders pose for a family photo during the European Union summit- the first one since Britain voted to quit- in Bratislava, Slovakia, September 16, 2016. REUTERS/Leonhard Foeger

European Union countries flags are seen at the Bratislava Castle (Hrad) during the European Union summit- the first one since Britain voted to quit- in Bratislava, Slovakia, September 16, 2016. REUTERS/Yves Herman

(L-R) Slovakia's Prime Minister Robert Fico, Finland's Prime Minister Juha Sipila, Swedens's Prime Minister Stefan Lofven and Lithuania's President Dalia Grybauskaite arrive to pose for a family photo during the European Union summit- the first one since Britain voted to quit- in Bratislava, Slovakia, September 16, 2016. REUTERS/Leonhard Foeger

European Union leaders pose for a family photo during the European Union summit- the first one since Britain voted to quit- in Bratislava, Slovakia, September 16, 2016. REUTERS/Leonhard Foeger