European Union leaders have agreed that member states should be allowed to pursue integration at different paces, days before Theresa May officially triggers Brexit.

The 27 other EU leaders were marking the union's 60th anniversary at an informal summit in Rome without the Prime Minister, as thousands joined an anti-Brexit march in Westminster.

European Commission president Jean-Claude Juncker called Brexit a "tragedy" while European Council president Donald Tusk called for sustained unity after Mrs May invokes Article 50 of the EU treaties to begin the UK's withdrawal on Wednesday.

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But in a declaration signed by the leaders to end the summit, the EU27 acknowledged that they could not always be fully united on all issues.

It stated: "We will act together, at different paces and intensity where necessary, while moving in the same direction."

Addressing the summit in an ornate hall on the ancient Capitoline Hill where the Treaty of Rome was signed on March 25 1957, paving the way for the formation of the EU, Mr Tusk said: ""Europe as a political entity will either be united, or will not be at all.

"Only a united Europe can be a sovereign Europe in relation to the rest of the world.

"Only a sovereign Europe guarantees independence for its nations, guarantees freedom for its citizens."

Italian prime minister Paolo Gentiloni admitted the EU had "triggered a crisis of rejection" as its pace of development slowed, but insisted "we stand together and we move forward".

It came as thousands joined an anti-Brexit march in central London, with Liberal Democrat leader Tim Farron, former deputy prime minister Nick Clegg, Labour MP David Lammy and Green co-leader Jonathan Bartley expected to address a rally in Parliament Square.

Former Ukip leader Nigel Farage predicted the EU was on the verge of breaking up.

"The EU calls for unity in Rome ring hollow as Brexit becomes a reality," he said.

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"The splits in the union on migration and the euro are leading to acrimony between member states as the North and South resent each other.

"The end is nigh."