* Supermarket retailers fall as survey shows market shareloss
* Fed tapering uncertainty also weighing on stock market
* FTSE 100 closes down 0.6 pct at 6,486.19 points
* FTSE still up around 10 pct since start of 2013
By Sudip Kar-Gupta
LONDON, Dec 17 (Reuters) - Britain's main equity index fellon Tuesday, with major supermarket groups hit by fresh signs ofa loss of market share, while uncertainty over future U.S.monetary policy also weighed on the stock market.
The blue-chip FTSE 100 index closed down by 0.6percent, or 36.01 points, at 6,486.19 points, retreating after a1.3 percent gain on Monday.
Supermarket retailers joined the FTSE's loserboard after asurvey by market researcher Kantar Worldpanel showed over halfof Britain's households had shopped at German discount grocersAldi or Lidl in the past 12 weeks.
"The top supermarket firms are losing market share at thebottom end to Aldi and Lidl, and losing out at the top end toWaitrose," said Hartmann Capital trader Basil Petrides.
UNCERTAINTY OVER FED TAPERING
A further factor weighing on stock markets was the U.S.Federal Reserve's meeting, starting on Tuesday and ending onWednesday, at which it may start to scale back the stimulus thathas driven much of this year's global equity rally.
The majority of investors and economists expect the Fed towait until March 2014 to begin winding down its "quantitativeeasing" (QE) programme. But stronger-than-expected U.S. economicdata, along with last week's budget deal in Washington, have ledsome to speculate the Fed may act this week.
"People have taken some gains off the table prior to the Fedmeeting," said JNF Capital trader Rick Jones.
He felt the pullback was a chance to buy stocks cheaply,however, and expected equity markets to rise again in 2014 asthe global economy gains momentum.
Jones expected the FTSE 100 to end 2013 in the 6,700-6,900point range, with the FTSE still up by around 10 percent sincethe start of 2013.
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