LONDON — Americans are drawing down their personal savings at the fastest rate since the depths of the Great Depression, suggesting that American household finances may be more fragile than they look.

The savings rate fell to minus 1% in 2006 and has now been negative for 21 consecutive months, according to Commerce Department data.

Such a rate was last seen in 1933, when a quarter of the American workforce was unemployed and whole families were kept alive by charitable soup kitchens.

There is no emergency to account for the slide this time. The American economy is well into a long boom that has shaved unemployment to 4.6%, a phase of the cycle that should normally lead to a strong rise in savings.

The chief strategist for Lombard Street Research, Charles Dumas, said a spending spree by rich Americans sitting on fat asset gains might have played a role, but the main driver was distress borrowing by households struggling to keep their heads above water as each source of stimulus dried up.

“There are no more tax cuts, no more house price gains, no more real income growth, and no more petrol price bonus,” Mr. Dumas said. “Step one is to carry on spending because that is the divine right of Americans, and step two is to run up second mortgages. But people are near the end of the rope and it’s no surprise that subprime lenders are now getting into trouble.”

The default rate on sub-prime loans reached a decade-high of 10.09% in November, according to a report by Friedman Billings Ramsey & Co. Some 16 lenders have already gone bankrupt.

The troubles are spreading to the capital markets where many of the loans are packaged together and sold as securities. These are then “insured” through the Credit Default Swap market.

The ABX index that measures these swaps has dropped 5% over the last six weeks, a substantial fall for an instrument of this kind.

Some $390 billion in mortgages with adjustable rates, taken out in 2004 and 2005 when interest rates were far below current levels, are due to ratchet up this year, in some cases doubling payments.

A senior economist at Standard & Poor’s in New York, Beth Ann Bovino, said American housing prices were likely to fall between 7% and 8% from peak to trough before bottoming out later this year.

“People are not going to be able to tap into their homes as cash machines any more and the ratio of debt service costs to disposable income is now at an historic high of 18.2,” Ms. Bovino said. “We’re not expecting the economy to fall off a cliff but household spending is going to have to slow down.”

Until last year, Americans were subsidizing their lifestyles to the tune of $70 billion a month through withdrawal of home equity. This has since dropped to nearer to $30 billion a month. The shortfall has been more or less covered by the falling savings rate — so far.

Yale professor Jacob Hacker said the average American was now walking a shaky financial tightrope, without a safety net. “American family incomes are on a frightening roller-coaster, rising and falling much more sharply than they did 30 years ago,” he said.

Mr. Hacker said personal bankruptcies had risen to more than 2 million in 2005 from under 300,000 in 1980.