The scandal over fixing the rate used as a reference for financial transactions worth more than $350 trillion globally has led to a fine for Britain's Barclays (BARC.L) and prompted an antitrust investigation by the European Commission as well as a pledge to criminalize the fixing of such indexes.

Michel Barnier, the European commissioner in charge of regulation, and Joaquin Almunia, the EU's antitrust chief, will face questions from the parliament's influential economic and monetary affairs committee from 9.00 a.m. EDT on Monday.

Gary Gensler, the chairman of the U.S. Commodity Futures Trading Commission, will testify by videoconference. Masamichi Kono, chairman of the International Organisation of Securities Commissions will also attend.

Barnier has raised the possibility that EU regulators could take over supervising benchmarks such as Libor, breaking with the current system where banks themselves supervise the London-based rate and its continental European equivalent Euribor.

He has already outlined a proposal for criminal penalties for those who rig an index such as the London Interbank Offered Rate (Libor), but new rules are not set to take effect until 2015.

"We want to find out how they intend to ensure that the criminal sanctions are sufficient to discourage the manipulation of such indexes," said Sven Giegold, a German member of the European Parliament.

"We want more," he said. "We want to go beyond the limited scope of the Commission's proposal."

Almunia, the European Commissioner leading the antitrust probe, has added to the pressure for reforms to the way benchmark rates are set with his probes into suspected rigging of Libor, Euribor and the Tokyo-baser rate Tibor.

Any banks found guilty of breaching EU antitrust rules could face fines of up to 10 percent of their global revenues. Sources have told Reuters that several lenders are providing information to Almunia's officials in the hope of securing lower penalties.

The Commission has not identified the banks, but sources familiar with the matter have said Deutsche Bank (DBKGn.DE) is cooperating with the EU regulator.

Barnier, who oversees European market regulation, is examining how such indexes are compiled.

His concerns are shared by the European Central Bank, which is privately pushing for a rethink on Euribor too, sources familiar with the matter told Reuters.

Libor rates are compiled from estimates by large banks of how much they believe they have to pay to borrow from each other.

But support is growing for changing the calculation to actual lending rates instead of the current system, with regulators worried that the existing set-up allows too much discretion.

Martin Wheatley, Managing Director of UK watchdog, the Financial Services Authority, will announce his recommendations for reforming Libor supervision and governance on Friday.

(Additional reporting by Foo Yun Chee; editing by Janet McBride)

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