The loan book includes the property-backed Clydesdale Bank loans of Scottish SMEs who have complained of severe pressure from the loans being called in.

Clydesdale transferred its portfolio in October to parent NAB, which said last week the book had been shrunk by £300m to a value of £4.8bn, excluding provisions for bad debts.

Clydesdale and Yorkshire banks told customers last July that their loans were no longer being managed locally, and agreements would not be renewed.

John Glare, spokesman for NAB Customer Support Group, said the portfolio was "essentially toxic".