In a report that accompanied a public questionnaire, the Commission spelled out its concerns about such financial benchmarks, which are used to set a standard across a market, and signaled its intention to introduce new restrictions on how they are compiled and implemented.

"The international investigations underway into the manipulation of Libor (London Interbank Offered Rate) have revealed yet another example of unacceptable behavior by banks," Michel Barnier, the European commissioner in charge of regulation said in a statement.

"Wider work is required to regulate how indices and benchmarks are compiled, produced and used."

The Libor rate, compiled from estimates by large banks of how much they believe they have to pay to borrow from each other, is used to determine interest rates on over $500 trillion of contracts around the world.

Libor was first established in 1984 and has been an industry standard since the late 1980s. Similar rate benchmarks - such as Euribor - have been set up in other financial centers.

Libor is being investigated by multiple authorities, including those in the United States, Britain and the EU, with investigators looking at a number of big banks that participate in the process of setting the benchmark rate.

Barclays was the first big bank to settle with U.S. and British authorities following allegations that its traders colluded with others to fix the rate.

(Reporting By John O'Donnell)

• A Permanent Economic Emergency