* Brazil makes boldest move so far to curb currency gains
* Measure aimed at reducing record short forex holdings
* C.bank sees short positions down to $10 bln vs $16.8 bln
(Adds currency futures, analyst comments, central bank estimate on forex short positions)
By Isabel Versiani and Ana Nicolaci da Costa
BRASILIA, Jan 6 (Reuters) - Brazil made its boldest move so far to curb gains in the real on Thursday, imposing reserve requirements on banks' short positions on U.S. dollars in a bid to reduce speculative trade.
The decision came just days after Finance Minister Guido Mantega said the new government would act to protect local manufacturers but does not plan any new taxes on foreign investments or other capital controls unless the real strengthens further.
The central bank imposed reserve requirements equivalent to 60 percent of whatever is smaller: a short position on U.S. dollars of $3 billion or the bank's reference capital. The measure will be effective as of April 4, the central bank said.
The surprise move will make it more expensive for banks to bet against the dollar. Short positions on dollars, a bet the real will strengthen against the greenback, surged to the highest on record at the end of December.
Brazil's currency weakened sharply on the futures market after the announcement, with the near-date real futures contract weakening 0.51 percent to 1.691 on news of the move.
"It should have a noticeable effect on the currency," said Luciano Rostagno, chief strategist at brokerage CM Capital Markets in Sao Paulo . "The government has been indicating that it was not happy with the real close to 1.65."
The real closed at 1.673 per dollar in local market trading on Wednesday. The real is the world's most overvalued major currency by some measures, according to Goldman Sachs.
SHORT POSITIONS
Brazil is one of a number of major emerging economies battling the impact of a flood of cheap cash due to ultra-low Western interest rates. The speculative flow has driven up their currencies and prompted fears it could weaken exports and competitiveness.
Mantega said on Tuesday President Dilma Rousseff's newly-installed government would address the damage caused by its strong currency by enacting tax breaks and trade protection, rather than attempting to artificially weaken the exchange rate.
Banks' short positions on U.S. dollars, a bet that the real will strengthen versus the dollar, have surged in recent months to $16.8 billion at the end of December, the highest volume since the central bank began keeping track of the data in 1994.
Central bank monetary policy Director Aldo Mendes estimated short positions should fall to around $10 billion after the measure.
The real gained 4.6 percent in 2010 and has surged by just over 14 percent since hitting a low in May of last year.
(Writing by Elzio Barreto; editing by Patrick Graham)