Speaking in London, Mr Darling recalled the shock he felt when then-US Treasury Secretary Henry Paulson informed him of the decision, reports Bloomberg.
"I just didn't think they would do it," he stated.
Mr Darling was attacked by Mr Paulson for blocking a deal to allow Barclays to take over Lehman Brothers but the British politician said he felt he had little choice in the matter.
"I thought the British taxpayers might have something to say about [a British firm] underwriting an entire American bank," he explained.
Earlier this month, it was announced that Ernst & Young is to be investigated for its role in the collapse of Lehman Brothers.
The firm was the auditor of the US bank and is now to be probed by the Accountancy and Actuarial Discipline Board.
By Asim Shah