For months, investors have tried to get a handle on the bank's exposure to demands that it repurchase soured mortgages, especially those sold to private investors. Earlier this year, Bank of America estimated that its maximum possible loss from such demands was $7bn to $10bn - above and beyond the $6.2bn already reserved for probable mortgage-repurchase losses.

But Bank of America's latest quarterly securities filing said it arrived at the $7bn to $10bn figures after reducing estimates by $4bn to reflect the significant legal hurdles faced by private investors in trying to gain access to loan files or even being able to sue.