By Barbara Kollmeyer, MarketWatch

MADRID (MarketWatch) -- London's benchmark stock index stuck to the flat line on Thursday, with investors not surprisingly sitting on the sidelines ahead of the outcome of the Scottish referendum as voters headed to the polls.

The FTSE 100 index was flat at 6,786.04, after closing down 0.2% on Wednesday, its third-straight losing session. Investor caution over the potential for a vote for Scottish independence has kept investors wary. The outcome of the referendum is expected early Friday. Read Scotland to call London's bluff over sterling

The British pound (GBPUSD) traded choppy against the dollar, last trading at $1.6295 versus $1.6280 in late North American action. Also read: How to protect your portfolio against a Scottish exit

A survey from pollster YouGov Thursday morning gave pro-union support a narrow lead, showing 49% of the 3,000 people polled between Monday and Wednesday want to keep Scotland in the union and 45% support independence. The rest were undecided or didn't know, YouGov said.

"If Scots do vote to leave, the market will likely imply a large premium on protecting against volatility, potentially testing the bounds of recent price swings," said David White, financial trader at SpreadEx, in a note to investors.

Unlike the Stoxx Europe 600 and other regional indexes, London stocks failed to eke out any gains from the U.S. Federal Reserve's pledge to keep interest rates near zero for a "considerable time".

Stocks to watch: Shares of TUI Travel PLC were a top gainer in London, up 3.8%. On Wednesday, Morgan Stanley upgraded the shares to overweight, citing benefits of the company's pending merger with TUI AG, more details of which were announced on Monday.

Close behind was EasyJet PLC , which rose 2.4% after the cut-rate airline said Thursday that it would increase its dividend payout more than 20% and buy 27 more Airbus Group NV jets.

Miners were giving back ground as gold(GCZ4) and other precious- and base-metal prices fell. In the wake of the Fed, a rising dollar knocked gold back to levels not seen since January. Shares of Fresnillo PLC fell 2%, Randgold Resources Ltd. dropped 1.5%, and heavyweight Rio Tinto PLC (RIO) slipped 1%.

Also on the losing side, Unilever PLC fell more than 1%.

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