The state could have saved €10bn-€15bn if the bank guarantee had been structured to lessen its exposure to Anglo Irish and Irish Nationwide, leading banking experts have calculated for the first time.

The estimates come after Patrick Honohan, the Central Bank governor, said in his banking report that aspects of the September 2008 guarantee provided to bank bondholders could be "criticised" for increasing potential costs to the state.

TCD associate finance professor Brian Lucey said that at least 50% of the €14bn Anglo owed bondholders in medium and other long-term debt before the guarantee could have been, following normal market practice, written down without damaging the reputation of Ireland.

Irish Nationwide in 2007 held €5.8bn in senior long-term debt.

"Even allowing for conservative write down, savings of €10bn could have been made. That's a third of annual tax take this year," said Lucey.

UCD Economics professor Karl Whelan said the government could have "in theory" saved all the money it put into Anglo and Irish Nationwide, or 50% of the sum if it were concerned about safeguarding Ireland's reputation in the bond markets.

But he warned the guarantee should have been structured to avoid its "principal negative effect" in creating "a wall of money" of more than €74bn in senior and interbank debt the guaranteed banks need to refinance by the end of September this year.

"That is half of GDP. It is the single largest threat," he said.

Peter Mathews, an independent banking and financial consultant, said that "with proper financial engineering" the government could have saved €15bn depending on the levels of discounts it applied in writing down the various levels of debt in the guaranteed banks.

Governor Honohan made it clear in his report that "an extensive guarantee" was required in September 2008. But he said "the extent of the cover provided, including to outstanding long-term bonds, can – even without the benefit of hindsight – be criticised in as much as it complicated and narrowed the eventual resolution options for the failing institutions and increased the state's potential share of the losses."