Spain has charged global consulting firms Roland Berger and Oliver Wyman with valuing the banking system's deeply troubled balance sheets.

Bank of Spain figures show the banks held problematic property assets, including loans and seized property, of €184 billion, 60% of their property portfolio at the end of 2011.

Doubts about the real value of banks' property-related assets persist, however. 

In a new banking reform, announced May 11, the government required banks to set aside another €30 billion in 2012 in case property-related loans go bad. That was on top of €53.8 billion in provisions required under reforms enacted in February.

In the latest reform, the government had also promised to name independent auditors to check the real value of the banks' assets.

"The aim of this initiative is to increase transparency and definitively clear up doubts about the valuation of banking assets in Spain," the Bank of Spain said in a statement.

The review led by Roland Berger and Oliver Wyman would have two parts, it said. The first, due to be released in the second half of June, would be a general valuation of banking balance sheets in Spain and their capacity to resist an adverse scenario, the Bank of Spain said.

The second "fundamental" part of the review would be to contrast how each banking group estimates the value and deterioration of its assets, the central bank said.

For this, the consulting firms would hire three auditors by the end of May, charging them with fieldwork on the quality of banking procedures in recognising and making provisions for impaired loans. The results of this work would be revealed "in the next months," the statement said.

Earlier, Spain denied that it needs any foreign help for its banks, which are staggering under the mass of loans that turned sour after a 2008 property crash.

"No external help of any kind is needed," Economy Minister Luis de Guindos told reporters at a forum in Madrid.

De Guindos said the government was open to the independent auditors' conclusions but pointed to the effort already made by Madrid, which he expected to be confirmed by an IMF report due in a few days.

The economy minister said Spain's fourth-largest bank, Bankia, which was nationalised this month to salvage its balance sheet, would need about €7-7.5 billion. That would mean injecting about another €3 billion in addition to €4.465 billion the state put into the bank during the takeover when it converted a loan to its parent group into shares.

Moody's Investors Service last week cut the credit ratings of 16 Spanish banks by one to three notches, citing the effects of recession and the Spanish government's own reduced creditworthiness.

Meanwhile Spain's Economy Minister also said that the recession-bound economy will shrink again in the second quarter of the year.

"The second quarter will show a fairly similar performance to that of the first quarter," Luis de Guindos said.

Spain's economic output contracted by 0.3% in the first quarter of 2012 and by the same amount in the last quarter of 2011.