Cameron and Juncker
Cameron with EC president Jean-Claude Juncker in Brussels. The UK PM said he will be 'battling for Britain' at the EU summit in BrusselsGetty

Fears of a Brexit and a mining sell-off weighed down the FTSE 100 on Thursday (18 February).

While PM David Cameron is in Brussels with "now or never" talks about EU reforms, investors are considering the possibility of the UK public voting to leave the bloc. Cameron said he was "battling for Britain" in talks with European Commission president Jean-Claude Juncker.

Pacific Investment Management Co (PimCo) put the chances of Britain leaving the EU at approximately 40% in a report published on Thursday.

"UK polls suggest a tight vote with a very high degree of uncertainty," PimCo portfolio manager Mike Amey said. "We assume the UK will vote to remain in the EU. However, we assign a probability of up to 40% to a Brexit."

Miners and commodity traders also affected the British blue chip index on Thursday, as volatility in natural resources and oil markets continued. Commodity company Anglo American surged earlier this week, but ended the day as the biggest fallers.

Oil prices climbed up to $35.44 per barrel (Brent crude), despite uncertainty surrounding a possible deal between six of the biggest oil producers. Russia and Saudi Arabia agreed on an output freeze, but the deal still depends on participation by Iran's oil ministry.

Sanjiv Shah, chief investment officer at Sun Global Investments, said: "Iran has said it would back measures to stabilise oil prices but would resist any plan to restrain its oil output in the face of pressure from OPEC producers. It claims any such measures would be 'illogical' as it plans to continue production until pre-sanctions levels are met again."

"From a big picture view, the key thing is that OPEC is now seriously looking at some measures to try to do something and this is supportive for oil prices," Shah continued.

The biggest riser on the FTSE, which outperformed most European stocks, was British Gas owner Centrica. The energy company reported a smaller-than-expected drop in revenue and profit and cut dividends, which boosted its share price.

FTSE 100 biggest risers*

Centrica +7.16
IAG +2.84%
Persimmon +2.60%
Berkeley Group +2.24%
Barratt Developments +2.10%

FTSE 100 biggest fallers

Anglo American -7.71%
Standard Chartered -5.33%
Barclays -3.81%
AstraZeneca -3.61%
Rio Tinto -3.26%

FTSE 250 biggest risers

Fidessa Group +19.70%
Poundland +19.61&
Indivior +19.61%
Allied Minds +14.62%
Pendragon +10.23%

FTSE 250 biggest fallers

Tullow Oil -11.36%
IPF -4.82%
Onesavings Bank -4.80%
Jupiter Fund Management -4.39%
Petrofac -4.08%

*Based on data provided by the London Stock Exchange after markets closed