(Reuters) - Iran's rial sank about 5 percent in trading against the U.S. dollar on Monday after the central bank said it would change the currency's official exchange rate, prompting fears of another devaluation. The rial was trading in the free market at around 21,510 per dollar, according to Persian-language currency tracking website Mazanex, down from about 20,440 on Sunday. Central bank governor Mahmoud Bahmani said on Sunday he would announce a change to the government's "reference rate" of 12,260 rials to the dollar "within the next 10 days", Iranian media reported. Iranian media speculated that the new reference rate might be between 15,000 and 16,000 rials. Most Iranians are unable to obtain dollars at the official rate and must instead use the free market, which is much more expensive.

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