The European Central Bank and Germany's Bundesbank are examining the legality of the ECB's new bond-buying programme, a German newspaper said on Tuesday, pointing to another possible barrier to moves to end the euro zone debt crisis.

German tabloid Bild, which did not give details of its sources, said ECB and Bundesbank in-house lawyers were checking what scale and duration the programme could reach before breaching EU treaties.

The newspaper said this meant there was a possibility that the issue could soon be referred to the European Court of Justice and added that the ECB and Bundesbank wanted to legally "arm" themselves for this scenario.

The report alarmed financial markets, which lauded the bond-buying announcement this month as the sort of bold action needed to end the crisis. The euro fell in response.

Bild said the background to this was controversy over the issue of whether the ECB bond-buying programme violates the ban in EU treaties of direct financing of state deficits.

The Bundesbank and the ECB would not comment on the story. ECB President Mario Draghi said earlier this month, when he announced the programme, that he was certain it abided by the rules.

"We are sure that we are acting within our mandate, that we are not violating Article 123," he told a news conference after the bank's monthly decision on interest rates. Article 123 prohibits the ECB financing governments.

The ECB said then it stood ready to buy unlimited amounts of bonds issued by euro zone member states, provided they put in a formal request for aid and fulfilled strict domestic policy conditions.

Jens Weidmann, head of the Bundesbank, was the sole dissenting voice in the ECB's decision.

The plan is designed to lower the borrowing costs of euro zone states such as Spain and Italy by buying their bonds, but it has stirred anxiety in Germany where some fear the ECB is venturing beyond its mandate and potentially exposing taxpayers to billions of euros in risky debt.

The ECB spent more than 200 billion euros in its first bond-buying programme, the Securities Markets Programme. The now dormant SMP was started in May 2010 under the previous ECB head, Jean-Claude Trichet.

Then, the 17-country bloc's central bank said that it was allowed to buy bonds in the secondary market to ensure the working of the "transmission mechanism" by which its decisions on official rates have an impact on borrowing rates across the euro zone and the economy as a whole.

(Reporting by Michelle Martin; editing by Mohammad Zargham and Patrick Graham)