The European Union should abandon its “utopian dreams” of ever-closer integration to combat rising Euroscepticism, Donald Tusk has said.
The president of the European Council said EU leaders should concentrate on practical measures such as reinforcing borders and a banking union.
Mr Tusk warned of “dramatic consequences” if Britain left the EU.
He urged Europe’s leaders to match the energy and endeavours of those who wanted to break up the union.
Meanwhile, Spain’s Prime Minister Mariano Rajoy has warned that the consequences of an exit vote in the UK’s EU referendum later this month would be “very negative for British citizens”.
In a speech in Madrid, he said people from the UK would lose their right to move freely, work and do business in the largest economic area in the world.
More than 300,000 Britons are resident in Spain, about 35% of whom are retired.
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There is no worse prospect for the European economy than the omen of a triumph of anti-liberal and Eurosceptic political forces, whether left or right,” Mr Tusk told business leaders in Brussels.
“We must and can avoid this scenario,” he said.
Mr Tusk chairs the meetings of EU leaders when they gather as the European Council in Brussels.
His latest plea for Britain to remain part of the EU club follows a warning by the Organisation for Economic Co-operation and Development that the UK’s economy would suffer “a large negative shock” if it left.
John Longworth, from the UK’s Vote Leave campaign, criticised the OECD’s report as “flawed”.
In May Mr Tusk said the only alternative to the EU was “chaos” and criticised high-profile Leave campaigner Boris Johnson’s comments comparing the EU’s aims to Hitler’s.