UPDATE 1-UK watchdog asks banks to check for ties to Panama Papers

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LONDON, April 7 (Reuters) - Britain's Financial ConductAuthority has asked 20 banks and other financial firms to checkif they have any ties to Panama law firm Mossack Fonseca, theregulator said on Thursday.

A massive leak of documents from the firm released this weekdetailed how the world's rich skipped through loopholes to parkcash in low-tax jurisdictions.

"We have written to the firms earlier this week," an FCAspokeswoman said.

Recipients of the letter have been given until April 15 torespond to the disclosure request.

More than 500 banks, their subsidiaries and branchesregistered nearly 15,600 shell companies with Mossack Fonseca,according to the International Consortium of InvestigativeJournalists (ICIJ). HSBC, Britain's biggest bank, andits affiliates created more than 2,300 in total, the ICIJ says.

HSBC has dismissed suggestions it actively used offshorestructures to help clients cheat on their taxes.

The bank has said the documents from Panama pre-dated athorough reform of its business model.

Spokespeople for HSBC and Barclays declined tocomment on the FCA correspondence.

RBS and Standard Chartered both said theywere reviewing their operations and co-operating fully withregulators on the issue.

News of the FCA's letter follows calls from Conservativelawmaker and chairman of Britain's Treasury Select Committee,Andrew Tyrie, for a thorough investigation of the trove andprosecutions "wherever possible".

"The government's, and HMRC's (Her Majesty's Revenue andCustoms), objectives - to collect the correct amount of tax andclamp down on illegality - require a great deal of transparency.The government will need to press for more," Tyrie said onWednesday. (Reporting by Huw Jones, Sinead Cruise, Lawrence White andAndrew MacAskill; Editing by Jason Neely and Susan Fenton)