The accounting mistake at Hypo Real Estate’s “bad bank” offshoot cuts Germany’s official expected debt level for 2011 by 2.6 percentage points compared with its previous forecast, to 81.1 percent of GDP. Last year’s debt level sank to 83.2 percent from 84.2 percent, according to the finance ministry.

Hypo Real Estate was the most prominent German victim of the 2008 financial crisis and the “bad bank,” FMS Wertmanagement, was set up last year as a vehicle for its troubled assets.

FMS acknowledged miscalculations in its accounts on Friday, but it remained unclear what exactly led to it previously overstating the debt it was carrying by a total 55.5 billion euros.

“We are working at high speed to clear up the matter, we take it very seriously and there’s no question that it is, to put it politely, very annoying,” Finance Ministry spokesman Martin Kotthaus said.

“We have a first idea ... what might have happened,” he said, but did not elaborate pending results of the investigation.

The ministry was to hold a telephone conference with the bank and auditors Monday and face-to-face meetings later this week.

Hypo Real Estate ran into trouble following Lehman Brothers’ bankruptcy filing in the autumn of 2008. The government nationalized Hypo in 2009.

Last year, the bank transferred troubled assets nominally worth some 173 billion euros into the “bad bank.”