The UK’s biggest banks are preparing to swap £80bn-£90bn of mortgage-backed assets for Treasury bills with the Bank of England – nearly twice as much as the central bank originally envisaged when it unveiled its scheme to unblock the frozen bank-lending market.

According to debt market sources, the banks have approached credit rating agencies about how to structure deals that will receive the triple A rating required for securities that lenders want to swap for Treasury bills that can then be used to raise cash. The move comes amid signs that lending in the interbank market is becoming particularly tight for banks that cannot post collateral to ensure their debt will be repaid.