* FTSE falls, underperforms rival European stock markets
* Uncertainty over Scotland vote holds back FTSE
* Traders see FTSE rangebound until Scot result is known
By Sudip Kar-Gupta
LONDON, Sept 18 (Reuters) - Britain's top equity index edged lower on Thursday, lagging gains elsewhere in Europe, as uncertainty over an independence vote that got under way in Scotland pegged back UK stocks.
Scots began voting in an independence referendum that will decide the fate of the United Kingdom, after opinion polls showed hundreds of thousands were still agonizing over whether to retain the 307-year union with England or back secession.
Five surveys - from YouGov, Panelbase, Survation, Opinium and ICM - showed support for the "Yes" campaign favouring Scottish independence at 48 percent, against 52 percent for the "No" campaign that backs Scotland staying in the UK.
The blue-chip FTSE 100 index fell by 0.2 percent, or 10.50 points, to 6,770.40 points in early session trading.
That compared with advances of 0.4 percent on Germany's DAX and 0.2 percent on France's CAC. European stock markets were lifted by the U.S. Federal Reserve's renewed pledge to keep interest rates ultra-low for a "considerable time".
Societe Generale equity strategists backed staying away from UK equities for now, in case the Scottish "Yes" campaign won.
"There are big nerves ahead of the Scotland result tomorrow. I don't really see the FTSE moving much until the result is known," said Hantec Markets analyst Richard Perry.
Perry expected the vote result to show Scotland favouring staying in the UK, and said any such result in favour of the UK could push the FTSE back up to the 6,850 point level.
Securequity sales trader Jawaid Afsar also expected Scotland to vote in favour of staying within the UK, but added that the FTSE would remain rangebound until the final result was known. (Reporting by Sudip Kar-Gupta; Editing by Catherine Evans)