A British exit from the EU would have "unforeseeable consequences" for European cooperation, European Commission head Jean-Claude Juncker has warned in an interview with German media.
Britain is to hold a closely-watched June 23 referendum on whether or not it should stay in the 28-nation bloc, and opinion polls are showing that the nation is still largely undecided on the issue.
In an interview with the Funke Mediengruppe press group to be published Monday, Juncker warned that a so-called "Brexit" would "surely have unforeseeable consequences on European cooperation, about which I absolutely do not wish to speculate about because I am convinced that Britons will make the reasonable decision."
"All Europeans want Britain to remain in the family," he said, recalling that the EU had struck a "fair deal" with Britain in February on reforms aimed at keeping it in the bloc.
Britain first joined the then European Economic Community (EEC) in 1973, and in a referendum two years later the public backed membership by just over 67 percent.
But the country has had a strained relationship with Brussels, opting out of key projects including the euro and the Schengen passport-free zone.
The EU this week cut its eurozone growth forecasts for the year, listing the danger of Britain leaving the bloc as among the risk factors weighing on the economic recovery.
The International Monetary Fund and the G20 group of the world's leading economies have also warned of the economic dangers of Britain leaving the EU, while the OECD last month said there was "no upside" to a Brexit.
Even the United States has weighed in, with President Barack Obama saying last month that EU membership magnified Britain's global influence. He also warned that if Britain did leave and wanted to sign a separate trade deal with the US it would go to the "back of the queue".