Michael Bloomberg says Brexit could hit UK business
Former New York mayor Michael Bloomberg says a Brexit vote on 23 June could leave the UK "disadvantaged".
He also said it would reduce the ability of Britain - and America - to "influence the dialogue" in Europe.
Mr Bloomberg told the BBC's Today programme that the UK might in the future fail to negotiate trade deals as beneficial as its current ones.
Leave campaigners said the EU helped "big businesses and fat cats" but did not "work for the British people".
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Mr Bloomberg's name appeared on a letter in the Financial Times on Wednesday, signed by a number of major multinationals investing in the UK, warning of Brexit dangers.
The letter, signed by executives from firms such as Airbus, Microsoft, Cisco, Hitachi, Mars, and IBM, warned that leaving the EU could "materially affect future investment decisions" by companies such as theirs.
It added that "if there is one thing we as investors don't like, it is economic uncertainty", and concluded that "as investors, it is therefore very much in our interest that Britain stays in the EU".
Mr Bloomberg, who founded the financial news and data empire that bears his name, told the BBC that "the UK would be disadvantaged compared to the situation they have now", if it voted to leave the EU.
"They [the UK] have a special relationship with the rest of the EU, they have the borders that they can control, unlike the rest of the EU, they have a trade surplus with the rest of the EU. They have some abilities to influence the dialogue, without which - they would, and America, which is my concern, would not benefit," he said.
"It's not for me to tell British people how to vote, it is for me to explain what, as the employer of 4,000 people in the UK... what it means for our employees, and what it means for our company and what it means for America. And then that has to go into the thinking of the British people, who have to do what they think is right for themselves and the other countries that they have relationships with."
Mr Bloomberg added that it would be in the interests of the EU to negotiate trade deals which did not have as many benefits for the UK as current ones, to discourage other EU members from leaving.
"We live in an international, global world. A very big part of the UK's economy is based on exports to the EU and to other countries, so they certainly have an interest in the health of those other countries, and those countries having a respect for the UK. That's what creates jobs here in the UK.
"The trade balances are dramatically weighted in favour of the UK in dealing with the EU, and if they were to drop out, it's really hard to see how they could ever negotiate a trade deal with the EU that would give them the benefits they have now. After all, the EU would have every interest to not give them those, so that nobody else would drop out of the EU."
Responding to the multinationals' letter, Employment Minister Priti Patel, who is campaigning to leave the EU, said: "Of course Brussels is good for big businesses and fat cats who care about their bonuses - they can afford to spend huge amounts of money on lobbyists and lawyers to help them stitch up the rules.
"But it is bad for smaller businesses and entrepreneurs.
"The British people will not be browbeaten into making a choice against their interests on 23 June."
On Wednesday night, both sides of the EU debate set out their stall in speeches to the CBI.
Former Tory leader Lord Howard criticised the business lobby group's warnings about the effects of a Leave vote, and said a "lack of democracy" in the EU was damaging businesses.
But former Labour chancellor Lord Darling said leaving the EU could trigger another UK recession.