Bradford & Bingley should only be nationalised as a last resort, the Lib Dem Treasury spokesman has said.
Vince Cable said if it was in "serious difficulty" the "best option" would be a takeover by another bank "without any involvement by the taxpayer".
But if B&B had to be nationalised the government should step in and "just get on with it", Mr Cable said.
Gordon Brown refused to comment on speculation about B&B after its share price plummeted to a record low.
On Thursday the lender announced plans to cut 370 jobs.
All three of the main RBS hit by Ј691m half-year loss ...
Northern Rock makes a Ј585m loss ...
Northern Rock unveils job losses ...
Depositor protection proposed ... credit ratings agencies have downgraded B&B's creditworthiness, citing its problems raising funds and the excessive exposure of its mortgage book to buy-to-let and self-certified borrowers.
But Richard Pym, the recently-appointed chief executive, insisted the bank was "strongly capitalised" and was "fit for purpose going forward".
Northern Rock merger?
The BBC's business editor Robert Peston said the Treasury, the Financial Services Authority and Bradford and Bingley were working to find a solution over the weekend, to announce before markets opened on Monday morning.
Full nationalisation, which could well include merging B&B with Northern Rock, was very much a last option - not impossible, but the final resort if all else fails, he said.
Mr Cable said it was imperative that whatever the solution, it must be decisive and there should not be a repeat of the delays following the crisis at Northern Rock.
"If the bank is in serious difficulty, the best option is if it were taken over by another bank and without any involvement by the taxpayer," he said.
"But the speculation in the national newspapers is that that won't happen and in which case it will effectively be nationalised, and that's what a takeover of Northern Rock actually means.
"I just hope this time the government doesn't faff around for four or five months getting this sorted out and just gets on with it."
Mr Cable said depositors should not panic because their money would be guaranteed by the government under the Financial Services Compensation Scheme.
The prime minister was asked about B&B during his visit to New York for a UN development meeting.
He said: "I wouldn't comment, and nobody would expect me to comment, on these speculative statements that are made in newspapers.
"I'm not going to comment on any individual company or building society."
In a speech to the UN, Mr Brown called for an end to "the age of irresponsibility" in the global financial sector.
(BBC)
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