HBOS yesterday wrote down the value of its equity stakes in UK housebuilders by half - or about £100m - because of the turmoil in the sector.

The bank has stakes in six UK housebuilders, including 50 per cent in Crest Nicholson, and 20 per cent in retirement homes specialist McCarthy & Stone.

HBOS had £4.2bn of total lending and investment to the sector at the end of May, of which £3.5bn was in loans. About 38 per cent of its corporate loan book is property-related, though only 3 per cent of this relates to housebuilders.

HBOS said its exposure was related to niche and less volatile sections of the market, such as retirement homes and social housing, rather than to "volume-led" housebuilders.

Tom Rayner, analyst at Citigroup, said in a note: "While housebuilder earnings are expected to fall, land bank values do provide some protection."

The revaluation in HBOS's stakes is in line with the fall in the listed housebuilders, who have lost more than half their value since the start of the year.

The HBOS outlook will compound the gloom of listed housebuilders, who are about to start valuing their land banks to evaluate how much value they have lost.

An industry rule of thumb suggests a plot of land loses four times more value than the houses built on it. A 36 per cent fall in land prices would make it inevitable that developers would have to write down the value of some plots.

Mark Clare, chief executive of Barratt Developments, said: "The formal test of writedowns will be in September when we release our audited accounts, but we will issue a range based on our own assumptions at our trading statement on July 10."

All builders are likely to write down some of their land, according to analysts. Barratt and Taylor Wimpey, the two most indebted housebuilders, are thought by some to be at risk of breaching debt covenants over the issue.