At 8.52am yesterday the HBOS chief executive Andy Hornby was staring at his plunging share price and finding it hard to believe his eyes. In a matter of minutes a spiv or spivs with a plausible scare story had, it seemed, succeeded in wiping £3 billion off the value of his bank.

It was an extraordinarily precipitous slide for such a blue-chip company. Sudden share collapses are only usually triggered when companies issue dire profit warnings. HBOS had said absolutely nothing.

Rumour, speculation and scuttlebutt alone had thumped the shares — helped along by a flurry of trader panic never far from the surface of the stock market in these jittery times.

The response from HBOS and regulators has been extraordinary. The lessons from Northern Rock, when news of its troubles was accompanied by near silence from the authorities, have been learnt.

Bank of England officials took the unprecedented step of ringing journalists to tell them on the record that HBOS had not asked for emergency funding. Mr Hornby, the most media-shy of bank bosses, stated categorically that his bank was rock solid.

And the Financial Services Authority did its bit, blasting away at trash’n’cash merchants — the stock market low-life who place “down bets” on share prices and then spread damaging false rumours to send the share price lower and so make a profit.

There’s no hard evidence that the rumours that hit HBOS yesterday were deliberately planted. But it wouldn’t be surprising. These days it’s as easy to bet on a share price falling as to bet on it rising. A whole new category of investors — hedge funds — has grown up to take such positions.

The FSA was making angry noises yesterday. The maximum penalty for such market abuse is seven years’ jail and unlimited fines. But the chance of a culprit being caught and prosecuted is minuscule.

Establishing beyond doubt that someone is responsible for starting a false rumour is near impossible. Simply passing on a negative rumour and trading on it is not enough so long as the propagator honestly believes it might be true. Establishing what is false is also a grey area. To say “Bloggs Bank’s balance sheet looks a bit weak” might be legitimate opinion but could still hit the shares.

Prosecutions have succeeded for abuse in the other direction. The “City Slickers” journalists were jailed after buying shares and then tipping them in the Daily Mirror. But there, in a black-and-white newspaper column, was the proof of wrongdoing. Whispers down an unrecorded mobile phone line are a different matter.

Banks are particularly vulnerable to adverse rumour, built as they are on depositor confidence. All banks lend for the long term but take deposits that can be withdrawn at no or short notice. No bank in the world could cope if all its depositors wanted their money back at once.

For centuries bankers understood their fundamental precariousness. They housed themselves in marble halls to give the impression of solidity and dressed in bowler hats to look conservative and prudent.

Their regulators understood too and made them keep huge quantities of deposits in cash or highly liquid securities such as government securities. They also insisted on strict limits on how much they could lend and to whom.

The relaxation of the rules over the past 50 years has given customers many benefits in the form of plentiful cheap debt and the bonus of no longer having to genuflect to an all-powerful bank manager. But the credit crunch of the past nine months has revealed that there is a cost.

Banks are being badly damaged by rumours. This new vulnerability is partly self-inflicted. Until Northern Rock, British banks hadn’t suffered a depositor run for 140 years. They forgot that confidence is everything.

By running down liquidity levels and by straying deep into lending areas they do not fully understand themselves, let alone are capable of explaining to shareholders, they have weakened their defences to the gossips and rumour-mongers.

Even the sudden new openness of banks this year hasn’t helped. They are disclosing more than ever before. The problem is that outsiders just don’t believe the numbers. Credit Suisse, a pillar of the Swiss elite, put out minutely detailed results last month, winning plaudits for its candour, only to apologise seven days later after finding it had overlooked another £1.5 billion of losses.

Among some City lawyers there is a growing conviction that banks are, one by one, being picked off by unethical short-sellers, that the abuse is organised. If that is the case, the perpetrators should be in no doubt about the seriousness of their crimes: they are hacking away at the very foundations of Western capitalism.

But the banks are not entirely blameless victims in such crime.

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