The EU's top watchdog has challenged the European Central Bank over privately disclosing sensitive plans for money printing to a group of hedge funds, giving them a head start over investors who found out only the following day.

Earlier this month, a top ECB official, Benoit Coeure, said at a dinner in London attended by hedge funds, academics and bankers that the central bank would accelerate bond buying in the coming weeks.

When his speech was released on the ECB's website on Tuesday morning, the euro fell sharply and stock and bond prices jumped. Many investors cried foul.

In a letter to ECB President Mario Draghi, the European Union's Ombudsman has now asked for a "detailed account of the incident", joining the ranks of those, from lawmakers to fund managers, who have expressed concern.

Ombudsman Emily O'Reilly, who keeps tabs on European Union institutions, called on the ECB to outline the "measures it has taken to avoid a similar incident occurring in the future, so as to enable me to ascertain whether there is any need for action".

The ECB's response, she said, should be given in public within a fortnight, offering the prospect of further embarrassment in Frankfurt over the preview of sensitive information to a closed circle.

Although the Ombudsman, who monitors for lapses in ethical or transparency standards, is unable to demand a response, the public nature of its request pressures the ECB to do so.

The ECB has said the delay between the speech's delivery and publication the following day was due to a 'procedural error'. It has since stopped its practice of sending speeches to journalists under embargo for publication at a set time.

"The ECB will respond to the Ombudsman and our answer will be public," a spokesman said on Thursday.

While it is not clear at exactly what time Coeure revealed the information on the bond buying, charts show the euro fell sharply against the dollar around the time he spoke. The reason for this move is unclear, as is whether anyone who heard the speech traded on it before the public release.

But the ECB's handling of such sensitive information has come under attack from lawmakers in the European Parliament, some of whom have called for a code of conduct for such meetings with investors.

Central bankers regularly speak at events, give briefings and attend meetings that are not open to the media. ECB officials alone go to hundreds every year, hosted by a wide variety of organisations.