The top share index fell back on Thursday, led by weaker banks and commodity shares as leaders of the world's biggest economies gather in France for a G20 summit set to be dominated by the threat of Greece collapsing out of the euro zone.

Eurogroup head Jean-Claude Juncker said on Thursday that the euro zone is working on a possible exit of Greece, saying it was essential that other members of the currency union would not be damaged in such an event.

The leaders of France and Germany, angered at Greece's shock move to call a referendum on its latest bail-out plan negotiated last week, told Prime Minister George Papandreou on Wednesday that Athens would not receive any more EU aid until it decides whether it wants to stay in the euro zone.

With the Greek referendum not due to be held until Dec 4, and a vote of confidence in the Greek government due on Friday which could throw that timetable out, markets are left with a very uncertain month ahead, and markets hate uncertainty, said Mic Mills, head of electronic trading at ETX Capital.

Falls by miners <.FTNMX1770>, energy issues <.FTNMX0530>, and banks <.FTMNX8350> accounted for over half of the FTSE 100 index's early morning decline as fears that Europe's debt crisis could unleash financial chaos prompted investors to shed riskier assets.

At 9:07 a.m., the FTSE 100 <.FTSE> index was down 19.05 points at 5,465.10, albeit bouncing off an early session low of 5,402.63, having closed 1.1 percent higher on Wednesday.

EARNINGS EYED

Aside from the euro zone debt concerns, investors also had a big batch of corporate earnings to digest on Thursday.

Life insurer Aviva was a top FTSE 100 faller, shedding 2.1 percent after seeing its sales fall in the first nine months of the year, reflecting tough conditions in its key European markets.

Insurance peer and fund manager Old Mutual also fell back, shedding 2.3 percent, as its third-quarter results showed a bigger than expected decline in funds under management.

But RSA Insurance added 2.4 percent as it said it was on track to meet its full-year goals as its saw net premium growth of 11 percent in the third-quarter.

Also on the upside, BT Group < BT.L> jumped 3.5 percent after unveiling better-than-expected core profits and sales in the second quarter.

And food ingredients firm Tate & Lyle was the top blue chip gainer, up 4 percent, after its above-forecast 38 percent jump in second quarter net profit.

As well as watching events at the G20 summit in Cannes, investors were also focussed on Frankfurt, where the European Central Bank will hold its first policy meeting under new President Mario Draghi.

I think there is too much focus on .... There is no change priced in but I think we are going to have indications of a cut before January. What is crucial is bond purchases, said David Morrison, market strategist at GFT Global.

On the domestic data front, October's Markit/CIPS services PMI report, due at 9:28 a.m., is forecast to show a reading of 52.0, down from 52.9 in September.

(Additional reporting by Fracesco Canepa; Editing by Greg Mahlich)