Shares in one of Britain's largest lenders tumbled another 30 percent Monday as customers, driven by fears of insolvency, made run on the bank and withdrew billions.

Treasury Secretary Alistair Darling sought to assure depositors that their money was safe, even as former U.S. Federal Reserve Board chairman Alan Greenspan warned of difficulties ahead in Britain's booming housing market.

Trading in the bank's shares was briefly suspended Monday morning, but not before they tumbled 140 pence to 298 pence ($2.81 to $5.98), on top of a 31 percent fall Friday. By late morning, shares hovered around 300 pence.

Northern Rock, Britain's fifth-largest mortgage lender, issued a profit warning Friday and Bank of England agreed to provide it with emergency funding.

The British Broadcasting Corp. reported Sunday that customers had withdrawn nearly 2 billion pounds ($4 billion) from Northern Rock accounts, though CEO Adam Applegarth refused to give a figure.

Speculation about a takeover ran rampant.

"The images of customers queuing up in the high street has done irreparable damage to the franchise," said Nic Clarke, an analyst for Charles Stanley & Co. in London.

"There is value in Northern Rock for a predator with a strong balance sheet but they would have to move quickly to save whatever is left of Northern Rock's reputation," he added.

Customers lined up outside of Northern Rock branches across the country on Friday. On Monday, dozens of customers waited outside for branches in Birmingham to open, and more than 100 waited in Leeds.

"With the Wall Street crash in the 1920s, people were frightened and reluctant to put their money into banks so they kept it under their mattresses," said Roy Hornsby, 69, who was part of a line of about 50 people in Newcastle.

"So you can understand why older people are going to want to take money out," he said as he prepared to withdraw some, but not all, of his money from Northern Rock.

Darling, like he did Friday, appealed for calm.

"Whatever happens, people can get their money out of the bank, they don't need to worry about that," Darling said in an interview on GMTV.

"The whole reason that we provide support if a bank gets into difficulties, like Northern Rock, is to help it get the money to tide it over these difficulties."

Northern Rock extended its opening hours to accommodate customers.

"The way to restore confidence is very simple _ it's business as normal, it's allowing customers to do exactly what they want to do," Northern Rock's Applegarth told BBC radio.

"The customers are perfectly entitled to take out their money. We have got their money, the problem for us is the logistics of getting to them."

Northern Rock's problems came against the background of signs of cooling in Britain's booming housing market.

In an interview published Monday in The Daily Telegraph, former U.S. Federal Reserve Board chairman Alan Greenspan warned that Britain was susceptible to some of the problems now roiling the U.S. real estate market.

"Britain is more exposed than we are _ in the sense that you have a good deal more adjustable-rate mortgages," he said.