Ratings giant Moody's has warned that a Brexit could automatically lead to the UK's credit rating being put on a negative outlook - because the "costs would outweigh the benefits" of an exit from the EU.
"In our view, a decision to leave the EU would be credit negative for the UK economy," said Kathrin Muehlbronner, a senior vice president at the company's sovereign risk group.
The organisation said it would consider assigning a negative outlook to the UK's Aa1 rating following a vote to exit, "pending greater clarity on the longer-term impact on the UK's economic and financial strength".
"Unless the UK managed to negotiate a new trade arrangement with the EU that preserves at least some of the trade benefits of EU membership, the UK's exports would suffer," it said.
"It would likely lead to a prolonged period of uncertainty, which would negatively affect investment, in Moody's view.
"It would also place a significant burden on policy-makers who would have to renegotiate the UK's trade relations with the EU and other countries and regions, as well as reconsider other areas such as regulatory and immigration policies."
But in a statement today it added that an EU referendum sooner, rather than later, will "help to alleviate some uncertainty" around Brexit.
"We consider it positive that the referendum will take place as soon as June, as a lengthy period of uncertainty of the part of firms and investors would damage the UK's economic growth prospects," said Muehlbronner.