In a letter to the remaining 27 members, which are increasingly divided between the east and west, the European Council President issued a rallying cry to “preserve unity”.
Donald Tusk, 60, said: “We can only confront today’s uncertainties if we act in unison.”
Hinting at a joint offensive against Britain during the summit, when members decide whether to move Brexit talks to trade, he said “European unity is our greatest strength”.
He added: “We need this unity in order to solve the migration crisis, to tackle unfair aspects of globalisation, to deal with aggressive third countries, to limit the damage caused by Brexit as well as to preserve the rules-based international order in these difficult times.”
The comments come at a time of sizeable divisions within the bloc on a number of areas.
The UK’s decision to opt for Brexit has exposed divisions within the group with their reaction and giving rise to questions over how to tackle the issue.
Austria’s People’s Party topped the polls at the weekend and looks set to for a coalition with the far-right Freedom Party (FPO) to secure a government.
Reports in the Polish press suggest the FPO could insist Austria joins the Visegrad group, a four-nation alliance of the Czech Republic, Hungary, Poland and Slovakia that is opposed to immigration, as a condition of forming a coalition.
The Visegrad group, in turn, has challenged the authority of the EU by refusing to accept migrant quotas.
Hungarian Prime Minister Viktor Orban said at the weekend: “I have to say firmly, that all the institutions of the EU have utterly failed.
“Neither the European Commission, nor the European Council, nor the European Parliament protected the Schengen Treaty.”
The issue of the EU’s supposed commitment to human rights has also been thrown into question over the issue of Catalonia and its attempt to break away from the rest of Spain and form an independent republic with the European Commission President Jean-Claude Juncker siding with Mariano Rajoy’a administration in Spain.
Mr Tusk also backed the idea of a two-speed EU – itself a contentious issue with some of the East European members – with Mr Tusk saying “member states could move forward more rapidly in specific areas while keeping the door open for those who want to join later”.
He also suggested the EU should “proceed step by step” and move forward “with speed, ambition and determination” in areas where there is agreement while adding in “other matters” there would need to be further preparation.
Mr Tusk also said the EU “should focus on practical solutions to EU citizens’ real problems”.
He urged the need for change “in order to bring back a sense of stability, security and predictability in people’s lives as well as faith in the future”.
During the two-day summit in Brussels Prime Minister Theresa May is due to give an update on the state of the Brexit negotiations where also on the agenda is the question of stemming the flow of migrants from Africa, the plan for a ‘Digital Europe’ and issues of security and defence.
Also up for discussion is the crisis regarding North Korea as well as the situation in Iran and Turkey.