By Max Colchester and David Enrich 

LONDON-- Royal Bank of Scotland Group PLC on Friday admitted there were errors in the data it submitted to last month's European stress tests, the latest mishap to hit an exercise aimed at rebuilding confidence in Europe's banks.

RBS, which is 80% owned by the U.K. government, said it had overstated the amount of top-quality capital it held. The adjustment wiped around EUR5 billion ($6.27 billion) of capital from the bank's balance sheet meaning it only just passed the stress test.

"We are examining how this mistake was made, and will be working with our regulators as we do so," said an RBS spokeswoman.

The error is the latest in a string of data problems that have hit the test, which was orchestrated by the European Banking Authority as a way of rebuilding trust in the embattled European banking sector.

Soon after the test results were published European Central Bank officials found an error in the 2013 capital ratio of Italy's Banca Monte dei Paschi di Siena SpA. The ECB and EBA also published different numbers for Deutsche Bank AG's litigation costs.

The Bank of England notified the EBA about the latest mistake on Friday, after RBS previously notified the Bank of England. "The error is being rectified as soon as it was detected," an EBA spokeswoman said, adding that she isn't aware of any other banks having made similar errors.

The RBS error stemmed from the way it accounted for the tax relief it gets from previous losses. This meant RBS artificially boosted its capital base. When applied to the "stressed scenario" of the EBA's test it meant RBS's capital ratio 5.7% compared with the 6.7% previously reported. The pass rate for the test was 5.5%.

Write to Max Colchester at max.colchester@wsj.com and David Enrich at david.enrich@wsj.com

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