In early business, the FTSE 100 is down 34.53 points at 5,932.27.
The pound also fell again on Thursday. Sterling lost 0.4% against the dollar to $1.4115 and fell  0.4% against the euro to â‚¬1.2564.
The US Federal Reserve last night said it would keep interest rates on hold with growth prospects uncertain.
The possibility of Brexit was one of the factors that led the US Federal Reserve to keep interest rates on hold, Chair Janet Yellen said.
(Open): The FTSE 100 starts Thursday trading with falls as traders remain weighed down by uncertainty about the global economy.