Even a temporary state role, however, will carry political and financial risks for a government already tarnished by the debacle, linking its fate to a shrinking mortgage bank at a turbulent time for Britain's housing market.

The Conservative party called for Darling to resign.

"He is politically a dead man walking," George Osborne, the opposition Conservative Party's economic affairs spokesman said in parliament after Darling announced the draft legislation.

Darling retorted: "He might have risen to the occasion rather than playing petty politics with the stability of the banking system."

The government also faces the threat of a drawn-out legal battle with disgruntled shareholders.

Shareholders in Northern Rock, who stand to lose all their investment, reacted with anger as the suspension of the bank's shares left them unable to sell their stock.

Jon Wood, founder of the bank's top shareholder SRM Capital, told newspapers he would consider legal action, while the shareholders' association said it would not accept a solution which could allow an eventual buyer to profit.

An independent audit is set to determine the value of the shares and how much the government will pay shareholders.

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Northern Rock has been put on the government's books, classified as around 90 billion pounds of public debt, and the focus will now shift to how soon a buyer or buyers can be found for its assets.

Analysts say that could be easier if lending slows over coming years, shrinking the mortgage book as many of its customers remortgage with other banks, and as the funding model becomes driven by retail deposits and not capital markets.

But the outcome will still depend on wider markets and could take years. Rolls-Royce, for example, took 16 years to refloat.

Darling, who himself has a Northern Rock mortgage, said he would welcome approaches. "We remain confident ... that the Bank of England will be able to get its money back," he said.

Opposition politicians blame Brown for the crisis, pointing to the regulatory framework he put in place a decade ago when he was finance minister under Tony Blair.

While the government has criticised Northern Rock's business model, Brown blames the bank's woes on a credit crisis that started with risky mortgage lending in the United States and has since spread throughout the world's banking system.

Financial groups such as Citigroup, Merrill Lynch & Co, UBS and Bank of America have all taken multi-billion dollar hits on bad mortgage investments, while Germany is rescuing lender IKB.

Shares in British banks rose on Monday, partly buoyed by Northern Rock. Analysts at investment bank Bear Stearns said a smaller Northern Rock, once one of the most aggressive competitors, could ease competition in the mortgage market.

Sterling, however, dropped as nationalisation worries added to concerns about the economy.