Losses for private investors on Greek debt in the second financing package for Athens are likely to be between 30 and 50 percent, rather than the earlier agreed 21 percent, euro zone officials said on Wednesday.

The euro zone is reviewing the terms of its second financing package for Greece, including the private sector contribution, because Greece is in a deeper than expected recession and market interest rates have changed since then.

International inspectors now do not expect Greece to return to growth until 2013, rather than the earlier forecast 2012, which, together with delays in structural reforms and privatization, increases Greek financing needs.

The additional costs will have to be redistributed between governments and private investors.

Euro zone leaders agreed on July 21 to provide 109 billion euros to Greece in new official financing, together with the International Monetary Fund until mid-2014.

In addition, under a voluntary debt restructuring deal, private creditors would end up taking a loss of about 21 percent in the net present value of their Greek bond holdings -- what is called a haircut -- contributing an estimated 50 billion euros on a net basis through mid-2014.

Four euro zone officials confirmed that a haircut of 30 to 50 percent for private investors was now under consideration, but said no final decisions or agreements have been reached.

It is still very much in the open and remains to be seen what the initial reaction of private investors will be, one euro zone official said.

A voluntary participation is the target, for now at least, and many feel strongly that we must avoid any risk of full default, the official said.

The haircut will be set at a level compatible with the voluntary nature of the private sector involvement, a second euro zone official said.

Losses for the debt would be 39 percent if current market prices for Greece's risk profile were used, according to Hung Tran, deputy managing director of the Institute of International Finance (IIF), which is helping arrange the private sector deal.

He said criticism that the 21 percent loss was not enough was based on inaccurately evaluating the deal. When the deal was agreed it used an assumed discount rate of 9 percent for the bonds, but the yield on Greek debt has since risen to about 15 percent.

If people properly and correctly -- and consistent with market practice in the past -- evaluate the deal, they should use current discount rates. If they do that it implies the NPV (net present value) discount is 39 percent, Tran told Reuters.

The second financing program for Greece, funded through the European Financial Stability Facility (EFSF), the euro zone bailout fund, is to replace the first 110 billion euro bilateral emergency aid package from 2010.

Sources said that therefore the disbursement of the next, 8 billion euro tranche of aid for Greece would most likely be the last one under the old program and the remaining 37 billion euros of undisbursed funds would be folded into the second financing program.

(Reporting By Jan Strupczewski; Editing by Toby Chopra; editing by Ron Askew)