The pan-European FTSEurofirst 300 index was down 1.4 percent at 1,289.95 points by 1137 GMT after hitting its lowest since early April and extending the previous session's 2.3 percent drop. The STOXX Europe 600 was trading 1.5 percent lower after setting its lowest in more than two months.

European shares came under further selling pressure following a risk-off pattern in global markets as investors positioned for central bank meetings this week and a referendum on Britain's EU membership set for June 23.

One poll out after the market closed on Friday gave the "out" campaign a 10 point lead, though two subsequent polls over the weekend painted a mixed picture over which side will win.

Bookmakers still believe that the referendum is likely to result in a vote to remain in the EU, although the probability of an "in" vote has dropped 10 percentage points from last week.

Before then, the U.S. Federal Reserve, Bank of England, Swiss National Bank and the Bank of Japan are all expected to hold monetary policy steady against a backdrop of caution heightened by the Brexit issue.

"We're in uncharted territory in front of the Brexit vote, and then there's also the Fed this week. So the wall of worry is quite high at the moment. All the banks are a little bit lower, and they're the ones which are likely to get hit," said Zeg Choudhry, managing director at LONTRAD.

"For the next two weeks, you've got to be slightly mad if you've not got your money in defensive stocks."

Cyclical stocks - sensitive to economic shifts - were the top fallers, with the auto and bank sectors down 2.0 and 1.6 percent respectively. Commodity stocks also came under pressure, with miners down 1.6 percent and oil and gas shares down 1.3 percent.

Brent crude prices dipped below $50 a barrel for the first time in a week, hit by a darkening economic outlook.

Among individual movers, mid-cap company G4S fell more than 5 percent after Sunday's Orlando nightclub shootings.

Omar Mateen, who killed 50 people at a packed gay nightclub in Florida on Sunday, was employed by G4S and had undergone company screening in 2013 with "no findings", G4S said on Sunday.

Traders and analysts said that it would dent G4S' reputation in the United States, where it generates a substantial proportion of its revenue.

Today's European research round-up [RCH/EUROPE]

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(Additional reporting by Atul Prakash; Editing by Gareth Jones)

By Alistair Smout