A spokesman for Santander, which owns British bank Abbey and is in the process of buying Alliance and Liecester, said: "The retail deposits and branch network will be taken over by Abbey, details of which will be revealed later."
Media reports suggest B&B's 200 branches and savings worth about 22.2 billion pounds (28 billion euros, 40 billion dollars) held by 2.5 million customers are being sold off.
The bank's loans, worth 50 billion pounds and including 41 billion of mortgages, would then be nationalised. Finance minister Alistair Darling is expected to make a statement before the markets open later Monday.
The move would see B&B become the latest casualty of the global financial crisis that has forced governments worldwide to step in.
As the Santander deal was announced, US lawmakers agreed on the details of an unprecedented 700-billion-dollar bailout for struggling Wall Street banks to avert the worst financial crisis since the Great Depression.
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