* RBS says investigations could have "material" impact
* RBS CEO Hester had initiated internal review
* Commerzbank sees "considerably negative" effect from probe
* RBS shares down 0.4 percent; Commerzbank down 1.5 percent
By Matt Scuffham and Jonathan Gould
LONDON/FRANKFURT, Aug 22 (Reuters) - U.S. authorities are investigating Royal Bank of Scotland and Commerzbank over possible breaches of sanctions on Iran, in a widening crackdown which has already cost Standard Chartered a hefty fine.
An RBS spokeswoman declined detailed comment on Wednesday but referred to disclosures published with the bank's half-year results earlier this month. These said RBS had initiated talks with U.S. and British authorities on whether it complied with economic sanctions on Iran, and that it could face a "material impact" from the investigation.
The inquiry raises the possibility of a substantial punishment for the part-nationalised British bank, which is also being investigated for its involvement in the Libor rate rigging scandal, ramping up pressure on Chief Executive Stephen Hester.
The United States first imposed sanctions on Iran more than 30 years ago but has tightened them in recent years as it tries to stifle Tehran's nuclear programme.
In the disclosures accompanying the RBS results on Aug. 3, the British bank said it had "initiated discussions with UK and U.S. authorities to discuss its historical compliance with applicable laws and regulations, including U.S. economic sanctions regulations".
These followed an internal review begun by Hester shortly after his arrival at the bank in 2008.
"The investigation costs, remediation required or liability incurred could have a material adverse effect on the group's net assets, operating results or cash flows in a particular period," the bank said. RBS had been making similar disclosures for the past 18 months, the spokeswoman said.
The scale of the transactions being investigated at RBS, which is 82 percent-owned by the taxpayer, was not clear.
The Financial Times reported on Wednesday that the U.S. Federal Reserve and Department of Justice were conducting the investigation, citing several people close to the situation. It cited a person familiar with the situation as saying one risk manager had already left the bank following the internal review.
A spokesman for the Federal Reserve said it could not "comment on supervisory matters pertaining to individual institutions". A representative at the Justice Department did not respond to a request for comment.
Germany's second biggest-lender, Commerzbank, also said in a regulatory filing that investigations by the United States into violations of sanctions on Iran and other countries could hold "considerably negative" consequences.
Commerzbank, which is 25 percent-owned by the German state, said U.S. authorities were investigating whether its dealings with Iran, Sudan, Myanmar, North Korea and Cuba had violated U.S. embargoes, and pointed out that other banks had paid large settlements to end such investigations.
"The financial impact of the procedure and its termination cannot be predicted and could exceed eventual provisions, which could have considerably negative consequences," Commerzbank said.
Commerzbank repeated on Wednesday that it had had no new business with Iran since 2007 and that it was too early to say what the financial consequences of the U.S. probes would be.
Washington imposed economic sanctions on Tehran in 1979 after Iranian students stormed the U.S. embassy and took diplomats hostage. Until November 2008 U.S. banks could process some transactions for Iranian banks or individuals provided they were initiated offshore by non-Iranian foreign banks and were on the way to other non-Iranian foreign banks.
The European Union has also imposed sanctions, including a ban on trading Iranian oil, due to fears that Tehran is trying to develop nuclear weapons. Iran says the programme is purely peaceful but the measures are making it increasingly difficult for Tehran to conduct business in U.S. dollars and euros.
The United Nations Security Council has also introduced more limited restrictions.
Standard Chartered agreed last week to pay $340 million to the New York bank regulator after it was accused of concealing $250 billion in Iranian transactions. The London-based bank joined a long list of lenders which have been punished for doing business with sanctioned states such as Iran and Cuba.
Barclays Plc, Lloyds Banking Group Plc, Credit Suisse and ING Bank NV have agreed to fines and settlements totaling $1.8 billion, while regulatory filings show that HSBC Holdings Plc is under investigation.
In 2010, RBS agreed to pay $500 million to settle similar allegations by U.S. federal authorities that ABN Amro, a Dutch bank RBS acquired in 2007, had violated U.S. sanction laws.
Analysts expect RBS to settle with U.S. and UK regulators over its involvement in the Libor scandal later this year. It has also endured problems on the domestic front, with a computer systems failure causing massive disruption to customers in June.
Shares in RBS were down 0.4 percent to 236.5 pence at 1120GMT. Commerzbank's shares were down 1.5 percent at 1.27 euros by 1120 GMT. The STOXX Europe 600 banking index was down 0.8 percent.