Investors around the world are racing to assess potential losses from an alleged $50 billion fraud by Bernard Madoff, the Wall Street trader arrested last week. Media reports say this is the highest ever fraud of its kind.

European, US and other institutions, including private banks and asset management firms, have been hit by the fraud. Here are some of the exposed firms, according to Reuters:

Grupo Santander. Spain's largest bank said its investment fund Optimal has a €2.33 billion ($3.05 billion) exposure to Madoff Securities.

Ascot Partners: According to a Wall Street Journal report, the fund where former GMAC chairman Jacob Ezra Merkin is a money manager has an exposure of $1.8 billion.

Access International Advisors - According to a report by Bloomberg, Access has an exposure of $1.4 billion.

Benbassat & Cie - Swiss private bank has an exposure of SFr1.1 billion ($935 million), according to Le Temps.

Union Bancaire Privee - Swiss bank that invests in funds of hedge funds has lost about SFr1 billion ($850 million), according to Le Temps, citing unnamed banking sources.

Man Group – the UK-listed hedge fund firm said in a statement today it had $360 million invested in funds advised or sub-advised by Mr Madoff.

BNP Paribas - France's largest listed bank said it has a potential €350 million ($464.3 million) exposure.

Reichmuth & Co - Swiss private bank said its fund of funds Reichmuth Matterhorn had an exposure to investments linked to Mr Madoff that amounted to about $325 million.

Nomura said it had a Y27.5 billion ($303 million) exposure related to Mr Madoff, but the impact on its capital would be limited.

Maxam Capital Management - The fund has lost about $280 million on funds invested with Mr Madoff, according to a Wall Street Journal report.

Pioneer Investments, part of Italian bank’s UniCredit. It is exposed to $280 million through its Primeo Select hedge fund, according to Bloomberg.

EIM Group - Le Temps reported that EIM Group, a fund of hedge funds, said it has a $230 million exposure.

Fairfield Sentry - The $7.3 billion hedge fund run by Walter Noel's Fairfield Greenwich Group had accounts with Madoff Investment Securities.

Kingate Global Fund - The $2.8 billion hedge fund run by Kingate Management Ltd had invested in Madoff Investment Securities.

UBS - The investment bank unit of the Swiss financial group has a limited and insignificant counterparty exposure, its spokesman told Reuters.

Benedict Hentsch - Swiss private bank said its exposure to products linked to Mr Madoff amounted to SFr56 million ($47 million), or less than 5 per cent of assets under management.

Bramdean Alternatives, the UK asset manager, headed by well-known fund manager Nicola Horlick, said almost 10 per cent of its holdings were exposed to Mr Madoff. Bramdean said it had two holdings that maintain trading accounts with Bernard L Madoff Investment Securities that represented 9.5 per cent of its net asset value at end-October.

BBVA - Spain's second-largest bank said no customers in Spain were exposed to the alleged fraud.

Other firms and institutions named were clients of Madoff, according to the CNBC television channel:

Tremont Capital Management; Palm Beach Country Club; Sterling Equities - The investment firm, owner of the New York Mets baseball team, Bank Syz, the Swiss bank dedicated to asset management - the Swiss private bank could not be reached for comment.