But growing signs of a chink in the sector's armour is focusing attention on the mutual movement's contingency plans, with consolidation now said to be firmly on the agenda.

As their stock market listed counterparts have revealed write-downs after write-downs amid the credit crunch, building societies have so far appeared relatively unscathed.

Their sizeable base of retail savings deposits has been much envied as the wholesale money market crisis has put funding lines in the spotlight. Meanwhile their conservative investment policies have shielded the sector from the so-called "exotic" US mortgage-backed investments that have lost banks so much money.

How much longer they can remain shielded from the market woes as the credit squeeze wreaks havoc is, however, becoming a moot point.

Where they were once the beneficiaries of the wholesale money market woes – gaining billions of pounds in savings in the wake of the Northern Rock fiasco – they now appear to be losing mortgage market share faster than the banks.

Building societies advanced net loans of just £580m in March, down from £1.8bn in the same month last year.

The value of mortgage approvals was also 42 per cent lower than a year earlier at £3.01bn, according to Bank of England data, which showed building societies are reducing their share of net lending at a faster rate than the overall market.

Meanwhile, the results of some mutuals are revealing that all is not rosy, with Derbyshire Building Society announcing job and branch cuts in March after posting a 47 per cent drop in profits after tax to £8.7m in 2007. And Yorkshire Building Society proved the sector has not entirely escaped US sub-prime-related write-downs, with news in March of a total £65m hit from soured investments and a 30 per cent drop in 2007 pre-tax profits.

As a steady stream of listed banks are lining up to call on shareholders for cash – with Bradford & Bingley the latest to announce a rights issue – questions are being raised over building society plans.

With no shareholders to turn to for a cash injection, how do they bolster their finances should funding requirements require?

A steadfastly bullish industry body, the Building Societies Association (BSA), refutes claims that the clampdown in building society lending reflects a sector under threat.

Director general Adrian Coles said: "Building societies are being careful – you can't carry on lending at the same rate if everyone else is withdrawing."

He adds that, contrary to popular belief, every building society was open for lending in March. "There's no doubt they are being being more cautious over the perceived riskiness of some types of customers, but I don't think building societies are in any sense feeling worse affected than other lenders."