Major U.S. bank Citigroup has warned staff it might rebalance operations away from the U.K. if the country votes to leave the European Union (EU) in this month's referendum, according to the U.K.'s Guardian newspaper.
Citi's U.K. head, James Bardrick, told staff a so-called Brexit might impact the business, in an email cited by the Guardian on Wednesday.
"A vote to leave the EU is likely to have implications for our U.K. operations. To continue to serve our clients and maintain efficient access to those markets currently enabled through the EU passporting regime, we would likely need to rebalance our operations across the EU," he said.
CNBC received no immediate reply on Thursday when it contacted Citigroup to see the letter and ask for comment.
Citigroup's main U.K. office is in Canary Wharf in London, but it also has offices in Derby in England, Belfast in Northern Island and Edinburgh and Glasgow in Scotland. The U.K. is the headquarters of Citi's Europe, the Middle East and Africa operations.
The bank employs more than 9,000 in the U.K., according to the Guardian.
The EU guarantees the free movement of labor, goods and services between its 28 member countries. Bardrick cited the U.K.'s links to the EU single market as one reason for having operations in London, along with the city's status as a global financial hub.
"The efficient movement of talent between EU member states is also important to Citi and many of our clients," he said.
To read the Guardian story in full, click here.