Knight Vinke, the activist investor, launched a fresh attack on HSBC yesterday, accusing Europe's biggest bank of flattering its US sub-prime losses by failing to write down $30 billion (£15 billion) worth of mortgage assets.

The broadside came as HSBC revealed a $3.2 billion first-quarter writedown on loans by its US business to poor Americans.

HSBC's investment bank also took a $2.6 billion writedown on credit investments for the first three months, pushing the group's total losses on sub-prime to $25 billion.

The bank was bearish on the outlook for the United States, predicting a recession as increasing numbers of Americans defaulted on home and personal loans in the first quarter. HSBC, a big lender in America through HSBC Finance Corporation (HFC), said that US house prices would continue to fall into 2009.

Michael Geoghegan, HSBC's chief executive, refused to call the end of loan impairments. “I would say we're in a lull at the moment. The growth in impairments has slowed a little, but that may well be seasonal . . . I can't say if it'll go up or if it'll go down,” he said.

Knight Vinke said that HSBC should have been gloomier about its own prospects. The fund manager, which has been agitating for HSBC to sell HFC, said that the group was the only large bank not to make a fair value adjustment on its loans to customers and other banks.

If HSBC accounted for the loans at their market value, they would be worth almost $30 billion less than $1,218 billion book value that the bank ascribes them, Knight Vinke said. Of that loss, about $23 billion comes from HFC. Taking the writedown would have pushed HSBC into a $5 billion loss last year instead of a $24 billion pre-tax profit, the fund manager said.

Other problems at HFC include the need to refinance at least $80 billion of its own loans in the next 30 months and a potential $10 billion goodwill impairment, which Knight Vinke said would force HSBC to pump in extra capital. HSBC has spent $60 billion on buying and supporting the US bank.

Douglas Flint, HSBC's finance director, said that Knight Vinke's claims were based on a misunderstanding of accounting rules. He said:

“Customer loans are accounted for differently to trading assets. We wouldn't be permitted by current rules to account for our loan book in the way Knight Vinke suggests we should.”

The bank said yesterday that 5 per cent of its US mortgages were two or more months overdue at March 31, compared with 4.2 per cent at the end of last year. Bad credit card debts rose from 5.8 per cent to 5.9 per cent.

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