By the time you read this article, every word of it may well have been reduced to irrelevant nonsense. I very much hope that it will be. I hope that at 8 o’clock on Monday morning the London Stock Exchange will make an announcement that Northern Rock plc has been acquired – perhaps for a derisory price of a penny – by one or more of the strongly capitalised banks from around the world that have long been interested in raising their shares of the British mortgage market.

Another possibility, which would equally discredit everything I am about to write, would be that interest rates in the London interbank market this morning suddenly fall back to normal levels, indicating that Northern Rock can again rely on this market, as it always has in the past.

Assuming, however, that neither of these happy events has spontaneously happened, we have to consider the opposite extreme. All the evidence over the weekend suggested that neither the Bank of England nor the Government was knocking heads together in the City, either to reopen the money market for Northern Rock or to find a corporate saviour. This detached approach was predictable after Mervyn King’s high-principled statement on bank bailouts last Wednesday.

Intellectually, this statement could not be faulted. Mr King was right to distinguish between bailing out an entire banking system and offering liquidity support for individual banks. He was right to favour transparent market-based solutions over political intervention and arm-twisting in smoke-filled rooms. He was right to insist that the Bank of England’s response to the summer crisis was more consistent with sound economic principles than the scattergun approach of the Federal Reserve and the European Central Bank.

In principle, all these arguments are perfectly valid. In practice, there is a serious risk that Britain’s controlled experiment in market-based central banking will go wrong – perhaps this week.

To see why, consider how a strictly market-based response to the Northern Rock crisis might evolve. What would happen in this crisis if all participants went to the logical conclusion in following their individual interests as they appear today?

Asking myself this question over the weekend, I suddenly got a queasy feeling of a kind I had not experienced for exactly 15 years – on Black Wednesday, when George Soros earned his title as “the man who broke the Bank of England” and 18 years of Tory political hegemony collapsed in one day. In financial markets, it is usually a mistake to follow any line of argument to its logical conclusion, because unpredictable events or interests usually intervene to turn a crisis or a triumph into some kind of fudge. But every few years, a market somewhere gets the bit between its teeth and goes all the way – Wall Street in 1987, the ERM in 1992, Russia in 1998, internet stocks in 2000. When this happens, Darwinian logic of short-term survival takes over and events move much more quickly than anybody expects.

Could Britain now be on the brink of such an ERM-style crisis? It is very unlikely. But when Darwinism takes full control of the markets, the logic of events can shift suddenly, as it did in 1992. What are the dictates of selfish logic for anyone with money today in Northern Rock? After reading the press releases and hearing the Chancellor’s statements over the weekend, it is clear that the Bank of England has not, in fact, guaranteed all deposits. It has offered to lend money “as necessary”, but this is a deliberately vague formulation. The legal position, by contrast, is clear: savings over £2,000 are no more guaranteed at Northern Rock than they were at Barings, BCCI or Equitable Life.

Of course, the Bank and the Government would be deeply embarrassed if small depositors lost money. But what about depositors with £100,000, or £1 million, or financial institutions with 100 times that much? Darwinism supplies an easy answer: it is rational for all substantial depositors to withdraw their money – and to do so at once.

What if this happens? Northern Rock, when it last reported, had £34 billion of customer accounts and deposits from other banks. To repay this money, it would have to turn to the Bank of England for what would probably be the largest loan ever advanced by a Government to a private company anywhere in the world. What would be the political reaction to such a loan? Especially when it turned out that Northern Rock’s collateral included buy-to-let and unsecured lending in a housing market on the brink of a freefall?

Would Alistair Darling authorise such a loan and face the political backlash? This is a question that he may soon have to face – and if he leaves any wiggle-room in his answer, the rush of money out of Northern Rock could turn into a tidal wave.

The Bank would then have to stand by its promise to support Northern Rock. But what would happen to other banks with similar business models? Would their depositors be reassured or would they start to worry that the Bank of England was reaching the limits of taxpayers’ tolerance and central banking prudence?

It seems probable that some would demand similar reassurance - and if they failed to get it, they would join the Rock’s savers in the handful of banks that are clearly “too big to fail”. If a bank run spread from Northern Rock to any other institution – even a very small one – the game would suddenly be up, just as it was on Black Wednesday.

The Treasury would have to offer blanket guarantees to all British banks. The Bank would have to slash interest rates and flood markets with money, as it did in 1992, in a desperate attempt to restore the normal levels of interbank lending whose failure triggered the Northern Rock crisis.

Just as John Major had to eat the words of his defiant speech to the Scottish CBI just days before the ERM collapse, Mervyn King would have to tear up last Wednesday’s carefully crafted statement. He would have to acknowledge that, while the principles of market economics are usually the best guide for a central banker, there are exceptional crises when the black and white distinctions drawn so lucidly in last week’s Bank of England statement must be replaced by a pragmatic grey. He might even have to acknowledge that in such exceptional crises, the decision ten years ago to split the Bank of England, spinning off its responsibility for banking regulation, might appear a mistake.

If such events happened, what would be left of Gordon Brown’s reputation for financial competence or of the Bank’s hard-won credibility and independence? No wonder I felt slightly queasy as the sun set yesterday, on the fifteenth anniversary of Black Wednesday, with Britain’s rulers waiting calmly for the market to solve their problems, while the roar of a tidal wave seemed faintly audible from afar.

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