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Leaving the EU would be “very, very damaging” for Britain and lead to the eventual break-up of the bloc, Virgin Group founder Sir Richard Branson has warned.

Reaffirming his pro-EU stance, Sir Richard says he hopes “sense prevails” when he comes to the public referendum on June 23 and the UK stays in.

“I think it would be a very, very, very, very sad day if British people voted to leave,” he told Sky News.

“I think it would be very, very damaging for Great Britain … and I think it would be the start, most likely, of the break-up of the European Union.”

His comments came ahead of prime minister David Cameron moving today to outlining details of last week’s deal with EU leaders, which paved the way for him to call the referendum on EU membership.

He claims the deal strengthens British sovereignty and the UK will be “safer and stronger” remaining in the EU.

But London mayor Boris Johnson revealed he will campaign to leave – arguing it is a chance “to vote for real change”.

The bosses of about half of Britain’s 100 biggest companies are preparing to back Cameron’s campaign to keep the country in the union, the Financial Times reported.