EU President Donald Tusk has warned that the European Union could implode after Brexit which Theresa May is set to trigger next week.

Speaking at a summit to mark the EU’s 60th anniversary, former Polish PM Tusk urged Euro leaders to fight to save the troubled union.

3 EU President Donald Tusk issued a rallying cry to Europe's leaders ahead of UK's exit Credit: Getty Images

He said: “Prove today that you are the leaders of Europe, that you can care for this great legacy we inherited from the heroes of European integration 60 years ago.

“Europe as a political entity will either be united, or will not be at all.”

With British Prime Minister May absent, the other 27 countries signed a new declaration on the Capitoline Hill where six founding states signed the Treaty of Rome on March 25, 1957.

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The EU is facing a string of crises on top of Brexit including migration, turgid economies, terrorism and populism which is sweeping across the continent.

After welcoming the leaders to the Renaissance-era Palazzo dei Conservatori, Italian Prime Minister Paolo Gentiloni said: "We have had 60 years of peace in Europe and we owe it to the courage of the founding fathers."

The original Treaty of Rome was signed by Belgium, France, Italy, Luxembourg, The Netherlands and West Germany to create the European Economic Community (EEC).

The new Rome Declaration that the leaders signed, using the same pen that was used six decades ago, proclaims that "Europe is our common future" in a changing world.

3 Malta's Joseph Muscat, Donald Tusk, Germany's Angela Merkel and Italy's Paolo Gentiloni pose for pictures ahead of the summit Credit: Getty Images

But it also enshrines for the first time a so-called "multi-speed" Europe, in which some countries can push ahead on key issues while others sit out, an idea pushed by France and Germany but opposed by many eastern EU states.

French President Francois Hollande said the message from Rome was, "we're stronger together," while German Chancellor Angela Merkel said that "a Europe of different speeds does not mean at all that there is no common Europe".

European Commission chief Jean-Claude Juncker insisted the EU could ride out recent storms.

3 Tusk listens to Pope Francis delivering his speech during a meeting with European Union leaders at the Vatican on Friday Credit: AP:Associated Press

"Daunting as they are, the challenges we face today are in no way comparable to those faced by the founding fathers," he said, recalling how the new Europe was built from the ashes of World War II.

The leaders met with the words of Pope Francis ringing in their ears -- on the eve of the summit, the pontiff warned that without a new vision, the crisis-ridden bloc "risks dying".

The White House meanwhile congratulated the EU on its 60th birthday in a notable shift in tone for President Donald Trump's administration, whose deep scepticism about the bloc has alarmed Brussels.

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