By Marc Jones

LONDON (Reuters) - World shares fell for a third day on Wednesday and gold dropped to a five-month low, as focus returned to whether the U.S. Federal Reserve will start to wind down its bond-buying stimulus as soon as this month or next.

The to-and-fro of when the Fed will begin to halt the flow of cheap dollars has dominated trading worldwide for months. A run of data ending in the main U.S. jobs indicator - non-farm payrolls - on Friday may yet tip the balance again.

Polls of analysts and traders still point firmly to the U.S. central bank holding fire until next March but some stronger data has reheated speculation it could move earlier.

That prompted investors to cash in gains from recent rallies, and despite a steadier start European stocks (.FTEU3) were on the slide again by midday after suffering their biggest tumble since August on Tuesday.

Still, Johan Jooste, head of fund manager Julius Baer's London investment office said it wasn't necessarily the start of the Fed's withdrawal that would drive the market's reaction.

"The whole market has been focused on when tapering will start, but what I would ask (incoming Fed chief) Janet Yellen is when does she perceive it ending.

"If the Fed starts in Jan or March I'm not so sure that's necessarily the big deal, but it's a bigger deal if it ends it quickly," Jooste added.

The worry is that the reduction in Fed support will be like a stab with a pin for recently inflated asset prices.

A sharp 2.2 percent fall for the Nikkei in Tokyo (NIK:^9452) as it recoiled from a six-year high had led Asia lower overnight, and with Europe wobbling again MSCI's world share index extended its losses for the day to 0.3 percent.

In the FX market, the dollar retreated against the yen and a basket of currencies (.DXY) despite a small uptick in benchmark U.S. Treasury yields.

Wall Street, however, was expected to see a small rebound after Tuesday's falls for the Dow Jones industrial average (.DJI) the S&P 500 Index (.SPX). (.N)

ECB TIME

Ahead of the European Central Bank's meeting on Thursday, mixed euro zone PMI data reinforced the differing fortunes of its main economies, though having only cut rates last month additional measures from the bank look unlikely for now.

One of Europe's best performing economies is currently the UK, but high-flying sterling saw its wings clipped as growth in Britain's service sector slowed a little last month, breaking a run of upside data surprises.

The specter of cuts in the Fed's bond-buying continued to cast a shadow over emerging market shares and currencies. They were among the hardest-hit when Fed Chairman Ben Bernanke first floated the prospect of "tapering" back in May.

MSCI's emerging market (.MSCIEF) spent a third day in the red, while the Indonesian rupiah weakened 0.9 percent to 11,975 rupiah per dollar after earlier falling to 12,000 to match a near five-year low touched last week.

William de Vijlder, chief investment officer at BNP Paribas said that while further sell-offs were likely in EM assets, they shouldn't be as dramatic as earlier in the year.

"We call it 'tapering echoes'. With each echo you have, the intensity of the noise fades," he said.

With Ukraine's under-fire President out of the country, there was a temporary lull in the political tensions that have rattled its markets hard in recent days.

The Thai baht also stabilized at around 32.26 baht per dollar after the country's navy chief ruled out a coup after days of anti-government protests.

AUSSIE, GOLD DOWN

Back in Europe, the euro was steady at just under $1.36 as the dollar back-pedaled, but fell to a two-month low against the Swiss franc as the euro zone data weighed.

A buoyant Germany was not enough to stop the 17-nation euro zone's private sector losing momentum in November, dragged backwards by a downturn in France - the bloc's second biggest economy - and a continued recession in Italy.

Down under, the Australian dollar saw its biggest fall since July as it plunged to a three-month low of $0.9025 after data showed its economy running slower than expected.

Oil prices jumped ahead of this week's OPEC meeting, with U.S. crude futures hitting a five-week high and Brent holding comfortably above $112 a barrel. (O/R)

But gold and silver, which like stocks have benefited from the U.S. stimulus because of inflation fears, traded near five-month lows. Gold last stood at $1,213 per ounce while silver traded at $19.04, having slipped to $18.94 on Tuesday.

"There is definitely an attitude of wait-and-see in other assets and more selling pressure in gold," Mitsubishi analyst Jonathan Butler said. "The next leg down is being awaited and the stimulus for that could be the (U.S. jobs) numbers on Friday."

(Editing by Catherine Evans)