Reuters could not confirm reports from other media about the duration of the proposed program. The Wall Street Journal said it would last at least one year. News agency Bloomberg said the purchases would run to the end of 2016.

The ECB declined to comment on any of the reports.

The duration is significant but also contested, because Germany wants to contain the scale of bond-buying.

A program starting in March and running for a year would amount to a total volume of some 600 billion euros, based on a purchase rate of 50 billion euros per month. If a similar plan ran until the end of 2016, it would surpass 1 trillion euros.

The six-member board, which met on Tuesday, forms the nucleus of the 25-strong policy-making Governing Council. The council meets on Thursday to decide whether to embark on QE - printing money to buy sovereign bonds.

Market expectations are sky-high for the ECB to announce a large-scale plan, despite entrenched opposition from Germany's Bundesbank and concerns in Berlin that such a program could allow spendthrift countries to slack off on economic reforms.

A Reuters poll of money market traders on Monday showed they expected the ECB to announce a 600 billion-euro sovereign bond-buy plan, though they also believed that would not be enough to return declining inflation to the ECB's target.

Euro zone consumer prices turned negative last month, sinking to minus 0.2 percent, far below the ECB's goal of just under 2 percent.

The ECB has already cut interest rates to record lows, begun buying private-sector assets and funneled hundreds of billions of euros in cheap loans to banks. QE is the last big policy tool it has.

By buying sovereign bonds, the ECB would show its commitment to buoying inflation. It would also generate a 'portfolio effect,' whereby investors move into other assets - some of them outside the euro zone - thereby depressing the euro.

(Reporting by John O'Donnell and Paul Carrel; Editing by Larry King)

By John O'Donnell and Paul Carrel