(Adds c.bank source, dealer quote, details)
MOSCOW, Dec 5 (Reuters) - Russia's central bank moved to widen the rouble's trading band on Friday, a central bank source told Reuters, a move that opened the door to a fourth devaluation within a month, and the currency weakened to 31.62 from 31.30 against its dollar/euro basket.
The move comes one day after Russia's gold and foreign exchange reserves posted their first weekly rise in two months, enabling Prime Minister Vladimir Putin to say the cash will help the economy weather the global crisis.
The central bank was forced to let the rouble depreciate in three one-percent steps against a euro-dollar basket last month as prices for oil, Russia's main source of foreign currency revenues, fell sharply.
The source in the central bank confirmed the regulator had widened the trading band. Dealers said the central bank had changed the level at which it intervenes to support the rouble.
"Looks like they have found a new level, otherwise they would not sell (dollars). I think it will be another 30 kopecks (0.3 roubles)," said Gazprombank's chief trader Viktor Kholoshnoi.
The central bank runs a managed float of the rouble, keeping it within a corridor against the basket, made of 0.55 dollars and 0.45 euros. Given low oil prices and persistent capital outflows, the rouble will tend to weaken after such widening.
Economists polled by Reuters last month expect the rouble to weaken further against the basket to 32.2 by the end of 2009. (Reporting by Yelena Fabrichnaya, writing by Gleb Bryanski; Editing by Ruth Pitchford)