The pound would also take a hit, dropping to $1.25 and €1.20.
In February the FTSE fell as low as 5536.97, 10 per cent off its current 6,185.79. The fall would be cushioned however by a boost to earnings from the weakness in the pound due to the number of FTSE 100 companies that bring in revenue from abroad.
The vote is currently still too close to call, with both campaigns expected to ramp up rhetoric as 23 June approaches.
A vote to remain could boost the FTSE by five per cent, UBS experts reckon.
The warning from UBS is the latest in a flurry of politicians, economists, and business leaders that have called for the UK to vote to stay in the Union.
Caroline Simmons, deputy head of the UK Investment Office at UBS Wealth Management, believes that the performance of the UK’s largest 100 companies’ shares could vary by over 15 per cent in a 12 month period depending on the outcome of the EU Referendum.
Should UK citizens vote to leave the European Union, we could see the performance of the FTSE 100 index fall by over 10 per cent. The stock market valuation could drop closer towards valuations seen during the 2012 euro crisis.
In late 2011 the FTSE 100 dipped under the 5,000 mark.
It's thought banks, insurance, listed REITS, homebuilders, and general retail and leisure are the types of domestic sectors which may react the most negatively to an exit from the EU.
Similarly, they could regain some lost ground under a vote to remain.
Read more: How the FTSE 100 has changed in 10 years
The FTSE 250 would be sent lower by the fall in value of the pound, as the mid-cap companies generate 50 per cent of their sales in the UK, compared with just 25 per cent for FTSE 100 sales.