The two firms have a big part to play in plans to stabilise the housing market
Drops in the share prices of the top US mortgage finance providers Freddie Mac and Fannie Mae have shaken confidence and hit shares in Asia and Europe.
Freddie Mac shares fell some 18% while Fannie Mae slipped 16%, pushing shares in banks and finance houses lower.
The falls came after Lehman Brothers said pending accounting rules could mean the two will have to raise $75bn (£38bn) to meet their commitments.
However, the Lehman report also said they may be exempt from the new rules.
Following the fall, the share prices in the two companies, which are known as government-sponsored enterprises (GSE), were at their lowest levels since November 1993 and July 1992 respectively.
Both of them have seen their share prices fall by more than 75% since last August a period during which they have tried to balance their attempts to stabilise the US housing market with their attempts to avoid making greater losses.
The value of their mortgage-backed securities have also plummeted relative to US government bonds, which, combined with the falling share prices, makes it harder for the GSEs to raise the money they need and more expensive for their debt to be insured.
The job of the two companies is to lend money to mortgage lenders and maintain the flow of housing finance.
'Snowball's chance in hell'
The accounting change identified by Lehman Brothers is supposed to make it harder for companies to keep their assets and obligations off their balance sheets.
Such a change hitting the GSEs does not have a "snowball's chance in hell of happening", according to Thomas Lawler, a consultant who used to be a Fannie Mae portfolio manager.
He added that the accounting issue "piled on to other folks' previous estimates that the companies might be forced to take [losses]" on sub-prime and other risky mortgage assets.
The problems for the lenders came as the US Senate edged closer to passing a mortgage rescue plan to save hundreds of thousands of homeowners from repossession.
The GSEs would play a big part in the plan, the centrepiece of which would help higher-risk borrowers with expensive mortgages to refinance into cheaper, government-backed loans.
The bill may be passed by the end of the week with enough support to override a promised veto from President George Bush.