Hassan Almrei arrived in Canada in 1999 seeking refugee status and was arrested at his Mississauga home after the Sept. 11, 2001, attacks in the United States. (CBC)Canada's spy agency says it failed to tell a Federal Court judge that an informant against a Syrian-born terror suspect failed a lie detector test, and that a second informant didn't undergo a polygraph examination that CSIS claimed was given.

Federal Court Justice Richard Mosley disclosed the information in two letters to lawyers for Hassan Almrei, a refugee claimant CSIS believes has links to al-Qaeda.

The revelations come less than a month after another Federal Court judge criticized CSIS for providing questionable evidence in another case.

On June 5, the spy agency told Federal Court Justice Simon Noël that a confidential informant who provided evidence against Algerian refugee Mohamed Harkat had failed a lie detector test.

Almrei, Harkat and three other Muslim men were released under strict terms of house arrest after being held on controversial national security certificates. CSIS believes all five men are Islamist extremists with past connections to terrorism.

Almrei's lawyer, Lorne Waldman, said the latest disclosures are grounds for quashing his client's security certificate.

"I think it's extremely serious, and we'll be seeking … a stay of proceedings," he said.

Waldman says he'll argue that the failure to disclose the information to the court constitutes an abuse of process.

Harkat's lawyer, Norm Boxall, said he's also considering asking the court to have his client's security certificate lifted.

"This does affect the credibility of the material that [CSIS] is putting before the court, he said.

Boxall said it's important to find out whether CSIS was simply careless when it failed to tell the court about information that could undermine the credibility of its informants, or whether the spy agency withheld the information deliberately.

"Now to see it occurring in a second case, it certainly leads to the inference there's a systemic problem at CSIS," he said.

There are five security certificate cases, and a Federal Court judge has ordered CSIS to review its evidence in all of them.