Vitor Constancio, vice president of the European Central Bank, played down worries in financial markets that the ECB will not be able to find enough bonds to complete its "quantitative easing" as planned.

Starting next month the ECB plans to buy 60 billion euros of debt securities a month until September 2016. Some investors worry that top-rated bonds are already in short supply--especially Germany's, which make up the largest individual chunk of the program. German government debt, or bunds, will account for just over a quarter of the purchases, or around EUR12 billion each month.

Mr. Constancio said the pool of available bonds for purchases is EUR4 trillion, a "deep" market with "a lot of investors."

"We don't anticipate that there will be such a problem," he said.

The German treasury says it expects to issue this year EUR147 billion of eligible bonds--those with maturities of two to 30 years--while EUR132 billion of bonds will mature, meaning net new bund issuance of just EUR15 billion for the whole year.

Overall, the ECB's plans mean it has to buy EUR215 billion of German government bonds between this March and September 2016--26 times more than the amount the German government bond market is predicted to grow over the same period, Morgan Stanley says.

Subscribe to WSJ: http://online.wsj.com?mod=djnwires