NEW YORK (Reuters) - Some 20 European banks including HSBC Holdings Plc have agreed to reimburse non-US investors for $15.5 billion of losses from convicted swindler Bernard Madoff's Ponzi scheme, but some Swiss lenders remain holdouts, a lawyer representing victims said.

Javier Cremades, who helped create a legal network to pursue investor complaints about Madoff, said lenders in France, Germany, Portugal, Spain and the United Kingdom are among the settling banks. Lenders in Switzerland have resisted in part because of that country's bank secrecy laws, he said.

"They feel very protected, and many investors outside have non-declared money," Cremades, president of the Madrid law firm Cremades & Calvo-Sotelo, told reporters in New York yesterday. "The Swiss system is the toughest one."

Yet he said settling ought to be "not expensive at all" for lenders that might otherwise face a "huge reputational issue" by holding out, "at a time confidence is not in abundance".

Differing disclosure rules may explain why some banks do not reveal payouts, as they might in the United States.

Daniel Fischer, a Zurich-based lawyer handling claims of some Madoff victims in Switzerland, did not immediately return a request for comment.

Cremades disclosed the $15.5 billion sum in a story published on Monday by The New York Times, saying it covered some 720,000 investors outside the US, or about 80 percent of the clients represented by the firms.

Spain's Banco Santander has admitted its clients may have lost 2.33 billion euros ($2.87 billion) with Madoff. HSBC spokesman Andrew McNamara declined to comment.

US victims have been less successful recovering funds, in part because many invested directly with Madoff, or used smaller managers known as "feeder funds" to invest with him.

There were only about 4,900 active accounts with Madoff when the fraud was revealed on December 11, 2008.

Cremades said some European banks may seek reimbursement through Irving Picard, the trustee liquidating Madoff's firm.

Picard has recovered more than $1.5 billion, but through May 21 had already identified 2,085 other valid claims by Madoff victims, totaling $5.45 billion.

Madoff, 72, admitted in March 2009 to running what prosecutors called a $65 billion Ponzi scheme. He is serving a 150-year sentence at a North Carolina federal prison. Five others have also faced criminal charges.