Santander, Spain’s biggest bank, has blocked public access to the website of its Optimal hedge fund management arm after telling investors in a US equities subfund they face potential losses of €2.33bn ($3.29bn) in the allegedly fraudulent securities operation run by Bernard Madoff in New York.

The website of Swiss-based Optimal, which had more than $10bn under management at the end of 2007 for institutions and wealthy individuals, had boasted at length about its fund managers’ close attention to risk control and due diligence.