Writing in The Times Mr Bloomberg says “a vote to leave is a risk” and asks “is the risk worth taking?”
Leave campaigners said the EU benefited “big businesses and fat cats who care about their bonuses” but did not “work for the British people”.
Mr Bloomberg’s name also appears on a letter, signed by a number of the major multinationals investing in the UK, warning of Brexit dangers.
This letter, which features firms such as Airbus, Microsoft, Cisco, Hitachi, Mars, and IBM warns that leaving the EU could “materially affect future investment decisions” by companies such as theirs.
It goes on to say that “if there is one thing we as investors don’t like, it is economic uncertainty”, and concludes that “as investors, it is therefore very much in our interest that Britain stays in the EU.”
‘Troubling’
Meanwhile, in his Times article Mr Bloomberg expands on his fears.
“I have carefully evaluated the question of Brexit and concluded the risks involved are troubling,” writes Mr Bloomberg, who founded the financial news and data empire that bears his name.
“No one can say for certain if an ‘out’ vote would shrink the financial services industry, which accounts for about 12% of the UK’s economic output and the bulk of our customer base.
“But in my conversations with CEOs of banks and other industry leaders, with rare exceptions, they see Brexit as a serious complication that could lead some jobs to shift to the continent over time. Some in Frankfurt and Paris are rooting for Brexit for this reason.”
‘Punishing rival’
Mr Bloomberg adds: “I also worry Brexit will leave our UK employees worse off.
“No one knows for certain how the UK would fare in trade negotiations with the EU, but we know Brussels would hold substantial leverage, given that the UK is far more dependent on the EU for exports than the EU is dependent on the UK.
“What price EU leaders would exact is impossible to predict, but deterring other countries from breaking away – not to mention the opportunity to punish an old rival – will likely discourage them from looking sympathetically upon Britain. Even if fair terms are secured, achieving them may take years, and families may feel a pinch well into the next decade.”
He said that as an entrepreneur these were risks that he found unsettling.
‘Stitch up’
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.
