Greece might need to cut its debt by an additional EUR15 billion even after it completes a deal now under discussion with private creditors for a 50% haircut in order to restore government finances to sustainable levels, people familiar with the matter said Wednesday.

The EUR15 billion is a rough first estimate provided the haircut talks are completed successfully and it may change, one official said. The officials added that there is now wide understanding that the haircut alone won't be enough to bring debt to the levels agreed in October.

Weak tax receipts and costs arising from Greece's economic slump have made it more difficult for Athens to service its debt. The budget deficit this year is expected to be close to 10% from an initial target of 8.5%, officials say.

A European Union official said the precise need for extra debt reduction will be determined after talks on the haircut are completed and inspectors representing Greece's so called troika of creditors--the European Union, the International Monetary Fund and the European Central Bank--check on Greece's progress in implementing austerity measures later this month.

Athens is expected to wrap up talks with banks next week to cut by half the EUR206 billion those banks hold in Greek bonds. A sustainability report in October said the move, along with official loans, would cut the debt to 120% of gross domestic product in 2020 from more than 160% currently.

This could put Greece's debt sustainability on the agenda of the Jan. 30 EU summit. Options could include lowering the interest rate on the bailout loans, and allowing Greece to buy back its bonds held by the ECB at discounted prices, a possibility the ECB is likely to resist.

Greece's efforts to reduce its debts will be further complicated by signs that its budget deficits will be deeper than predicted in October, when the debt-reduction deal was agreed.

As a consequence, the second bailout currently being negotiated for Greece may need to be bigger to fund the shortfall, according to an EU official.

The ECB has never confirmed the amount of Greek bonds it has bought outright, but market estimates put its face value at anywhere between EUR40 billion and EUR60 billion. The bank bought them at a sharp discount to face value and could theoretically allow Greece to retire part of its debt by selling the bonds back to the government at the price originally paid.

The ECB has so far refused to be an active participant in any effort aimed at debt reduction, and only tolerated the euro zone's insistence on private holders of debt taking a loss in return for extra guarantees against losses on the collateral it holds in respect of lending to Greek banks.

"The ECB has not changed its position. It will not participate in the [bond] exchange. It's called the 'private-sector involvement' plan and the ECB is not private sector," a troika representative said.

Officials expect the agreement on the haircut to involve an exchange of old bonds for new ones with maturities ranging between 20 and 30 years and a coupon of 4% to 5%. The new bonds will be governed by English law, which means creditors could in theory seize Greek assets if the country fails on its payments.

The first official said the EUR15 billion in extra debt reduction assumes a full participation of private creditors in the haircut.

Athens is seriously considering using so-called collective-action clauses, or CACs, that would force a minority of holdout creditors to take losses, if the agreement is backed by a majority.

The Greek government will retroactively introduce CACs to its existing bonds to achieve this, according to several people familiar with the situation.

Greece will retrofit the bonds with CACs to gain leverage but will only use them if the participation level in the deal is deemed to be too low.

The EU is prepared to support the forced restructuring under these circumstances, according to an EU official. This constitutes a significant departure from the voluntary principle of the restructuring that had originally been agreed on in October 2011.