The European Central Bank is weighing up whether to allow the Greek bonds held in national euro zone central banks' investment portfolios to be subjected to the same writedowns private investors are set to take, central bank sources told Reuters.

Euro zone central banks hold around 20 billion euros (16.6 billion pounds) of Greek bonds in their traditional investment portfolios.

The bonds are separate to the 40 billion euros worth of debt bought by the ECB under the purchase programme it started in May 2010 and is now in the process of ring fencing to protect from writedowns.

Three euro zone central bank sources said that the issue of taking losses was currently under debate at the ECB.

One added that it was fifty-fifty whether the deal would go ahead, adding that the only time limit on a decision was the Greek debt deal with the private sector which is expected to be sealed early next week.

If central banks do take losses on the investment portfolio bonds it would provide an immediate lump sum for Athens and be another contribution from the ECB on top of the 12-15 billion euros of purchase programme profits that it has made clear will also be made available.

(Reporting by George Georgiopoulos and Marc Jones in Frankfurt, editing by Mike Peacock)