The underwriters of Bradford & Bingley's (B&B) rights issue, which was labelled a partial flop after investors took up only 28 per cent of the shares, have agreed not to dispose of the remainder of the shares for 20 days.

Citigroup and UBS had until 4.30 pm today to come up with the cash for the 597,263,479 new shares, some 72 per cent of the rights issue, or find subscribers for the 55p shares.

But B&B said in a statement: "The underwriters have informed the company that they are of the opinion that it is unlikely that any subscribers can be procured."

Under a rescue plan orchestrated by the Financial Services Authority (FSA) last month, Citigroup and UBS, alongside six of Britain's biggest high street banks, HSBC, Lloyds TSB , HBOS, Barclays, Santander’s Abbey and Royal Bank of Scotland, rallied round to sub-underwrite the capital-raising that was first announced in May but was delayed twice.

B&B shareholders handed over £455 million in exchange for less than a third of the shares earlier this month.

Shares were trading at just over a deeply discounted 55p last night, where they have languished for much of the week, dipping to 51.5p yesterday afternoon. They fell to 51p this morning.

The decision comes just days after Richard Pym, former chief executive of Alliance & Leicester, took over as the chief executive of Bradford & Bingley. He has pledged to try and extricate the lender from an agreement with GMAC to take on £350 million-worth of British mortgages each quarter from the American finance group until the end of 2009.

B&B will announce its interim results next Friday.

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