MOSCOW (Reuters) - The Russian central bank allowed the fifth one percent devaluation in the rouble against the dollar/euro basket in a month yesterday as fresh data showed a $17.9 billion drop in gold and forex reserves last week.

The rouble weakened as far as 31.89 from 31.63 against the basket, made of 0.55 dollars and 0.45 euros, at the start of the trading session. It closed at 31.84. The rouble is down 8.3 percent against from historic peaks set in early August.

The reserves fell to $437 billion on December 5 from their peak of $597.5 billion on August 8 when Russia sent tanks to Georgia, triggering a wave of capital flight. Since then the central bank has been selling dollars to support the rouble.

Yesterday dealers said the regulator did not need to sell dollars to defend the new support level as investors bought roubles on expectation there will be no more moves this week.

A source at the central bank confirmed the regulator had widened the rouble's trading band to the basket, used to guide the exchange rate policy. In an environment of low oil prices and capital outflows such widening leads to a weaker rouble.

Finance Minister Alexei Kudrin told reporters he fully backed the central bank's policy while the bank's Chairman Sergei Ignatyev declined to comment. The central bank has kept a low profile since the start of the depreciation in November.

The move followed the release of the third quarter GDP data which showed the growth slowing to a three-year low. Analysts have said that with the price of oil also at a three-year low, the gradual depreciation policy was not working.

"If forex reserve depletion continues at the current rate, and oil prices fail to rally, the Russian authorities are likely to undertake a more aggressive, one-off move to weaken the rouble," said Timothy Ash from Royal Bank of Scotland.

Ash argued such move may come around a 10-day New Year and Orthodox Christmas holiday next month when banks will be closed, which will help avoid a run on deposits in commercial banks.

Russian officials including Prime Minister Vladimir Putin have said there will be no sharp fluctuations of the rouble.

A stable currency is viewed as one of the most significant achievements of Putin's eight years as president. But officials admit the rouble must adjust to lower oil prices.

"This quarter and the next quarter the authorities will see how badly the economy is hurt and that should, if anything, speed up the depreciation," said Citi analyst Elina Ribakova said. A weaker rouble should boost economic growth.

Russia aims to fully float the rouble within three years as part of transition to inflation targeting regime.

"In our view, it would make sense to speed up the widening of the trading band and let the market help find a reasonable level for the rouble, thus avoiding draining forex reserves excessively," said Lars Rasmussen at Danske Bank.

The central bank is taking advantage of a euro at a six-week high versus the dollar, which makes the devaluation moves against the basket less obvious for the general public, who closely watch the rouble/dollar rate.

Thus, while the rouble has weakened by a total of around five percent versus the basket since devaluations started exactly a month ago, it has lost only two percent against the dollar.