The European Commission wants external audits carried out at Slovenia's biggest state-run banks before the euro zone country starts transferring some 3.3 billion euros (£2.8 billion) of non-performing loans to a newly-established 'bad bank'.

"The bad bank is ready, but the problem is that Europe does not believe the Bank of Slovenia's estimates of the size of bad loans in the banks, so it will take another week or so before transfers will start," Prime Minister Alenka Bratusek told parliament on Wednesday.

Nova KBM said the transfer of bad loans would begin after approval from the Commission. Officials say it fears Slovenia's central bank has underestimated the size of the problem.

The Bank of Slovenia had said it was sure the external audit will show its estimate of bad loans in local banks was correct.

"The audit of bad loans by an independent auditor is expected to start in July," Nova KBM said in a statement to Reuters.

With state banks nursing the lion's share of some 7 billion euros in bad loans, Slovenia had planned to start the transfers to the bad bank on June 28, but the European Commission has yet to give its approval.

Bad loans in the ex-Yugoslav republic shot up when Slovenia's exports hit a wall during the global financial crisis and the once thriving economy slid into recession.

The transfer of bad loans is one of a number of crisis measures the coalition government of Bratusek is pursuing, along with the privatisation of more than a dozen state-controlled companies.

The delay may fuel fears of slippage in the government's efforts to stabilise Slovenia's finances.

"We will help the economy most if we enable banks to finance the economy again," Bratusek said at the start of a budget debate.

Slovenia's three biggest lenders - unlisted Nova Ljubljanska Banka (NLB), Nova KBM and Abanka Vipa - will receive 1.1 billion euros in state-guaranteed bonds in exchange for the bad loans.

The government also plans to inject some 1 billion euros of fresh capital into the three banks in the coming weeks to ensure they meet European capital standards. It has slated Nova KBM for sale.

The Finance Ministry said last week the results of external stress tests of No. 1 lender NLB are expected in mid-July.

($1 = 0.7821 euros)

(Reporting by Marja Novak; Editing by Matt Robinson/Ruth Pitchford)

By Marja Novak