UBS, the Swiss investment bank, announced its largest quarterly loss of SwFr 12.5 billion (£5.7 billion) today after a multibillion-dollar write-off on US sub-prime mortgage assets, as America's Federal Bureau of Investigation (FBI) opened an investigation into 14 companies, including banks, in connection with the US home loans debacle.

In a statement released this morning, UBS said that its fourth-quarter figures would include a $12 billion (£6 billion) write-off of US sub-prime debt and a further $2 billion writedown of its US residential mortgage assets. The writedowns were 40 per cent higher than the $10 billion figure the bank highlighted at its profit warning in December.

Analysts at Citigroup said UBS could make further writedowns, pointing to the $2 billion loss on its non-sub-prime US residential mortgages as an area of ongoing concern.

Total losses for the year are expected to reach SwFr 4.4 billion. Shares in UBS fell 1.54 per cent to SwFr 46.08 in early trading.

UBS will report its fourth-quarter and full-year figures on February 14 — almost a year to the day that the US mortgage crisis first broke when HSBC gave its first profit warning in its 142-year history.

Its US sub-prime home loans division, which HSBC acquired when it bought Household International for $14 billion in 2003, was responsible for bad debts at HSBC rising to $11 billion, above the $8.8 billion expected by analysts.

Since early last year, UBS has written off a total $18.4 billion on its sub-prime assets. Like many of its US rivals, UBS has sought external investment from sovereign funds.

Late last year, the bank said that the Government of Singapore Investment Corporation would inject SwFr11 billion for a stake of about 9 per cent. Most recently, UBS wrote to shareholders to try to secure support for a SwFr13 billion (£6 billion) capital injection from a Singapore sovereign wealth fund and a mystery Middle East investor.

In America, the FBI said that its investigation into sub-prime home loans would look into claims of accounting fraud and insider trading.

According to The Wall Street Journal, the inquiry is part of the FBI’s decade-long investigation into US mortgage fraud.

It will concentrate on stages of the securitisation process, from the companies that packaged sub-prime mortgages up into bundles of debt to the banks that held the assets.

While the FBI declined to name the 14 companies it was investigating, it did say that they included "developers, lenders and financiers".

The FBI said that it was working closely with the US Securities and Exchange Commission (SEC), the country's regulator, which itself has opened three dozen investigations into the sub-prime mortgage market.

Goldman Sachs and Morgan Stanley admitted in their annual reports yesterday that they had been requested to submit information on their respective sub-prime mortgage activities to authorities but declined to name which organisations had requested the data.

Morgan Stanley also acknowledged that it was the subject of litigation in connection with the mortgage crisis.

Federal prosecutors in New York and the SEC are also looking into the collapse of two Bear Stearns hedge funds as well as New Century Financial, a mortgage lender that is in bankruptcy.

The FBI said that it was looking into the financials of companies that have gone into bankruptcy as part of the mortgage debacle.

Sharon Ormsby, the head of the financial crimes section in the FBI's criminal investigative division, said: "The bundling of these loans is so huge, it's difficult for us to tear apart each of those bundles and say, 'OK, this bad loan is causing this securitisation to fail'."

She added: "We have observed that sub-prime loans are decreasing but the suspicious activity reports we see ... have noted that suspicions of mortgage fraud are increasing."

Meanwhile, BNP Paribas, the largest French bank, said that it expected its fourth-quarter net profits to fall by 41.8 per cent to €1 billion (£742 million), stating that the global credit crunch had impacted earnings to the tune of €309 million.

BNP Paribas contributed to the problems across the world’s big financial centres when, on August 9 last year, it announced that it was freezing three of its investment funds in the wake of the US sub-prime mortgage crisis.

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