James Saft is a Reuters columnist. The opinions expressed are his own.

james-saft1Even in the good times, many British consumers were borrowing against their houses just to fund routine consumption, indicating a big hit to come for retail sales and for the banks who hold the loans.

With house prices falling rapidly and mortgage debt tougher to get, it is no surprise that homeowners are less able and inclined to borrow against their houses in order to spend.

That will be hitting the High Street now - analysts are expecting a 0.6 percent fall on the month in retail sales for November when data are released later this week. But a rise in unemployment next year could expose a really serious weakness in household finances, as consumers who counted on being able to extract wealth from their houses to smooth consumption in bad times find that, when bad times come, the wealth isn't there and the banks don't want to lend anyway.

Researchers at Durham University looking at survey data found that 37 percent of homeowners borrowed against their house between 2002 and 2005, typically realising about 6,000 pounds. That's a lot people borrowing a lot of money against very illiquid and now hard to realise assets.

Even more interesting is the pattern of what householders were doing with the money and what was happening to them when they decided to borrow. Over time the proportion of people borrowing to re-invest in their houses through improvements fell, while more was finding its way into day-to-day costs, according to Susan J. Smith, a professor at Durham and one of the authors of the study.