Credit Suisse today became the latest bank dragged into the escalating “dark pool” trading scandal.

The Swiss bank, which employs 7500 people in London, is now the fourth alongside Barclays, UBS and Deutsche Bank to face US probes over the way the pools have been operated.

So-called dark pools are supposed to allow big investors to trade anonymously and so be protected from the market moving against them when they try to move large parcels of shares.

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But US authorities are concerned over the way allegedly “predatory” high-frequency traders may have been allowed in.

Credit Suisse said it was “responding to inquiries from various governmental and regulatory authorities concerning the operation of its alternative trading systems, and is co-operating with those requests.”

While the bank gave no further details, Eric Schneiderman, the New York Attorney General, is known to be investigating the bank. Barclays has sought to dismiss a lawsuit he brought against its dark pool operation, claiming his evidence was shaky and that he had pushed fraud laws beyond their limits.

UBS and Deutsche Bank revealed earlier this week that they are being probed by US regulators.

The Credit Suisse admission came on a black day for European banking with both troubled Espirito Santo — whose financial difficulties have sparked fears that they could tip off a fresh crisis — and French bank BNP Paribas announcing record losses.

Banco Espirito Santo posted a net loss of €3.6 billion (£2.9 billion) blaming “extraordinary events” for €4.25 billion of impairments and contingency costs.

With its capital buffers devastated by the charge, the bank will need fresh cash from investors but could yet draw on emergency funds held by the Bank of Portugal.

BNP’s woes were expected after the $9 billion fine imposed by the US last month for sanctions busting. It was €4.32 billion in the red in the second quarter. But the bank was able to retain its capital at sufficient levels to satisfy regulators thanks to otherwise strong underlying results.