The blue-chip FTSE 100 index <.FTSE> closed down 0.3 percent to 5,922.12 points, after falling to 5,899.97, its lowest since late February.

Investors were also cautious after the Federal Reserve cut its growth forecast and Fed Chair Janet Yellen acknowledged Britain's possible EU exit would have consequences for economic and financial conditions globally.

"Usually traders will push equity markets higher if the Fed is going to stay on the dovish side, but today is not that day," Naeem Aslam, chief market analyst at TF Global Markets, said.

"Brexit anxiety has its own flavour and it is very bitter."

Opinion polls have shown unexpectedly strong support for Britain leaving the EU in the June 23 referendum, prompting concerns about the impact on the bloc, the unity of the United Kingdom and the wider impact on global financial markets.

Polls by Ipsos MORI and Survation on Thursday showed the "Out" campaign in the lead, confirming a recent trend that has shown the momentum is with those who wish to leave the EU.

Both sides of the debate suspended campaigning in the afternoon after a British Member of Parliament was left in critical condition having reportedly been shot and stabbed in her constituency.

Cyclical stocks were the worst hit, with the UK banking index <.FTNMX8350> falling 0.9 percent and touching a two-month low. Shares in Royal Bank of Scotland (>> Royal Bank of Scotland Group plc), Barclays (>> Barclays PLC) and Lloyds (>> Lloyds Banking Group PLC) fell between 1.3 and 2.6 percent.

"If there was a vote to leave the EU, the immediate reaction in banking stocks would be quite dire. We've had an atrocious performance in banks quite broadly so far this year, and if economic growth is set to be hit ... then you get out of banks," said Chris Beauchamp, market analyst at IG.

Some mining stocks also came under pressure, with Anglo American (>> Anglo American plc) and BHP Billiton (>> BHP Billiton plc) down by 3 and 1.8 percent respectively.

The FTSE 100 ended off its lows, however, after Wall Street and commodity prices partially recovered from initial falls.

Gold producer Randgold Resources (>> Randgold Resources Limited) rose 4.8 percent, the top gainer in the FTSE 100 index, to its highest in nearly two years on wider economic uncertainty.

Among other movers, 3I Group (>> 3i Group plc), Mediclinic International (>> Mediclinic International PLC) and Severn Trent (>> Severn Trent Plc) fell 2.6 to 3.6 percent after their shares traded ex-dividend.

(Editing by Catherine Evans)

By Atul Prakash and Alistair Smout