"There is a risk that the 66 billion pounds invested in RBS and Lloyds may never be recovered," Margaret Hodge, chair of the Committee of Public Accounts, warned in a report into the sale of taxpayer-backed Northern Rock.

Northern Rock, a much smaller business based mainly on mortgage lending, was sold to Virgin Money last year with an estimated loss of 469 million pounds on the U.K. taxpayers' investment, after its "bad debt" was spun off. The committee of MPs said that the total loss to the taxpayer over Northern Rock was around 2 billion pounds, after alleged mismanagement of the bank when it was first nationalized under the Treasury, then led by Alistair Darling as Chancellor of the Exchequer.

"The Treasury was unable to respond promptly when the banking crisis hit because it lacked the right skills and understanding. It was slow to nationalize the bank and that made a loss difficult to avoid," Hodge said.

"After nationalization, it then failed to effectively challenge the optimistic business plan put forward by the bank's management to split the bank."