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By John Revill

ZURICH, Aug 23 (Reuters) - Swiss National Bank Chairman Thomas Jordan could be out of action for several weeks as he recovers from a heart operation over the weekend, a source told Reuters, a situation which could see him miss the central bank's next interest rate decision.

Jordan had to undergo a urgent medical procedure at the weekend, which proved necessary after a check-up, the SNB said on Monday.

The 58-year-old, the architect of the SNB's ultra expansive monetary policy over the last six years, is still in hospital and not expected to be back at work for several weeks while he is being treated for a heart condition, the person familiar with the matter said.

It is not clear whether he will return in time for the SNB's next monetary policy assessment on Sept. 23.

In early trading, the Swiss franc firmed slightly against the euro, reaching 1.0734 against the single currency.

"The operation was successful and Thomas Jordan is in a good condition," the SNB said in a statement. "He will be able to devote himself fully to SNB official business again after the medically recommended period of convalescence," it added.

Jordan has been chairman of the SNB since 2012 and was last year reappointed for another term until 2027.

Until his return, governing board members Fritz Zurbrüegg and Andréa Maechler, along with their deputies, will deal with the bank's affairs, the SNB said.

Economists expect the SNB to maintain its policy of using the world's lowest interest rates of minus 0.75% and a readiness to intervene in the currency markets to head off appreciation pressure on the Swiss franc.

Apart from being a little more active in the foreign exchange markets in recent weeks, the SNB has shown few signs of deviating from the policy in place since it suddenly quit pegging the franc to the euro in January 2015.

"We don't expect any change to the SNB monetary policy because of this," said Karsten Junius, an economist at J.Safra Sarasin. "I would wish Mr. Jordan all the best and have no doubt that the SNB continues to run perfectly in his hopefully short absence." (Reporting by John Revill; Editing by Tom Hogue, Michael Shields and Tomasz Janowski)