A U.K. vote to leave the European Union could see the FTSE 100 fall by over 10 percent in the next year, according to an analyst at UBS Wealth Management.

Caroline Simmons, deputy head of the U.K. investment office at UBS, said the shares of the U.K. benchmark could vary by over 15 percent in a 12-month period depending on the outcome of the EU referendum, which is being held on June 23.