The crisis engulfing the world’s financial markets moved closer to home yesterday when Britain’s biggest savings group was hit by a selling stampede.

Regulators and HBOS itself, which owns Halifax and Bank of Scotland, rushed out statements to reassure its 15 million savers and stave off the unthinkable – a flood of customers trying to withdraw their money.

HBOS shares fell by 41 per cent at one point, after a top ratings agency questioned its financial strength, and closed at 182p, down 22 per cent on the day. The price had already fallen by 18 per cent on Monday.

In America, the insurance giant AIG, which sponsors Manchester United, was teetering on the brink. David Paterson, the New York Governor, who is coordinating the rescue effort, gave warning that AIG had just one day left to resolve its financial problems before triggering “catastrophe”. A collapse of AIG would plunge the credit crisis to new depths, since every leading bank has exposure to it.

Shares around the globe slumped for the second day amid fears that more financial institutions could be allowed to fail. The FTSE 100 index of British blue-chip companies slid below 5,000 for the first time in five years. Central banks worldwide pumped $160 billion into money markets after a $36 billion injection on Monday.

One banker in London said: “Today is the day of fear and capitulation. It could be the bottom [of the crisis], but only if AIG gets saved. If not, then there’s going to be anarchy.”

George Soros, the hedge fund supremo, said: “I’m afraid we’re not through the worst of it at all – in some ways we’re heading into the storm, rather than coming out of it.”

The Treasury, Bank of England and Financial Services Authority called an emergency meeting to consider measures to cope with the unfolding crisis, which was triggered by the collapse at the weekend of the investment bank Lehman Brothers, owing $613 billion.

It is understood that Alistair Darling, the Chancellor, is ready to issue a depositor guarantee for any leading UK financial institution should this prove necessary. He said that Britain’s financial markets were experiencing a period of “almost unprecedented turbulence”. Action was being taken but governments and regulators could not guarantee to “stop the huge economic forces and changes that are taking place at the moment”, he added.

Financial advisers said that depositors should not worry because it was unthinkable that the Government would allow ordinary savers to be left out of pocket even in the highly unlikely event of HBOS failing. Last November the Treasury stepped in to guarantee all Northern Rock deposits.

The Financial Services Compensation Scheme guarantees the first £35,000 of an individual’s savings in each banking group.

Investor sentiment was soured by a hard-hitting report from Standard & Poor’s, the agency responsible for assessing balance-sheet strength, which said that HBOS was “less well positioned to manage the deteriorating operating environment” than its peers. Some analysts are worried about toxic investments on HBOS’s balance sheet, including £6.6 billion of securities whose value depends on poor borrowers in America with impaired credit histories. The bank has also been picked on because of its relatively high dependence on funding from the wholesale money markets.

HBOS has argued that it is strong and that it has been able to fund itself easily. It has grown its deposit base by 12 per cent this year and boasts £258 billion of savings. It says that its core Tier 1 capital ratio, the most common measure of bank strength, is the highest among UK domestic banks.

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