UPDATE 1-Iran unifies official and open market exchange rates as rial hit new low

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LONDON, April 9 (Reuters) - Iran unified the country's official and open market exchange rates, state media said, after its currency, the rial, plunged to an all-time low on Monday on concerns over a return of crippling sanctions.

The U.S. dollar jumped in a day from 54,700 rials to 60,000 rials in the open market in Tehran on Monday. A dollar was worth 36,000 rials in mid-September.

After an emergency cabinet meeting, Iran's First Vice President Eshaq Jahangiri was quoted by the state media as saying that from Tuesday the price of the dollar would be 42,000 rials in both markets, and for all business activities.

Iran has long been trying to unify its open market rate, used for most commercial transactions, with the official rate, which is a subsidised rate that is only available to government departments and some importers of priority goods.

Jahangiri said from Tuesday the government would not recognise any rate but the official rate, and "it would be illegal to trade dollars with an unofficial rate."

U.S. sanctions lifted under Iran's nuclear deal with world powers in 2015 will resume unless U.S. President Donald Trump waives them again on May 12. Trump has effectively set that as a deadline for European powers to fix what he called "the terrible flaws" of the deal.

President Hassan Rouhani warned on Monday that Trump will regret it if he pulls out of the nuclear deal. (Reporting by Bozorgmehr Sharafedin; Editing by Adrian Croft)