Ken Clarke said the deportation case against Russian Katia Zatuliveter, which used the controversial closed hearings, collapsed “presumably on the grounds that she may have been an embarrassment but she was not a security threat”.
Defending his widely-criticised plans for more “secret courts”, the Minister without Portfolio said the reforms in the Justice and Security Bill were about national security alone.
The specialist lawyers involved in such hearings launched a “ferocious” attack on the Government’s plans to extend the use of so-called closed material procedures.
The special advocates, who are appointed to represent someone’s interests when they are excluded from hearing evidence, said key safeguards were missing and the plans would create a statutory straitjacket for judges.
But Mr Clarke said they were underplaying their own effectiveness, citing Miss Zatuliveter who won an appeal to remain in the UK last November after the Security Service argued she was passing information to Moscow.
She embarked upon an affair in 2006 with MP Mike Hancock, and went on to work for him in Parliament where he sat on the Defence Select Committee, before she was arrested in December 2010.
But the Special Immigration Appeals Commission (Siac), which used hearings behind closed doors from which even Miss Zatuliveter and her lawyers were excluded, ruled in her favour.
Speaking at the Policy Exchange think-tank in central London, he added: “A lot of people say special advocates are far more effective in practice than they claim they were.”
The former Justice Secretary also said the chance of the plans leading to more secret hearings being used in cases like Hillsborough and Jimmy Savile were as likely as him picking up his trademark Hush Puppies and running a marathon.