A federal judge on Tuesday barred James Cole, a former deputy U.S. attorney general, from continuing to defend the Chinese telecom Huawei, siding with federal prosecutors who argued the Sidley Austin partner should be disqualified based on his role in an unspecified investigation during his tenure at the Justice Department.

U.S. District Judge Ann M. Donnelly's decision to disqualify Cole came almost six months after federal prosecutors in Brooklyn argued that an "obvious conflict of interest" should prevent him from representing Huawei, which is facing charges it violated sanctions against Iran.

Donnelly did not immediately make her decision public, instead submitting it to a "classified information security officer" for a review. The judge said the decision would be made public— with redactions—by Jan. 10 and directed Sidley Austin to take "prompt and reasonable steps to address all issues arising from Mr. Cole's conflict."

Cole, who served from 2011 to 2015 as the second-ranking Justice Department official, was not immediately reached for comment about the ruling. Huawei continues to be represented in the case by Jenner & Block partner David Bitkower and Sidley Austin partner Michael Levy.

In extensively redacted papers, federal prosecutors did not publicly specify the past investigation that purportedly presented a conflict of interest, but they raised concerns that Cole could use information from the probe to help Huawei. Prosecutors said the court could have "no confidence that Cole will not use, whether intentionally or not, information" from that investigation.

Describing the U.S. government as Cole's "former client," prosecutors said ethics rules "protect a former client against such conduct by a former attorney."

Cole entered an appearance in Huawei's case in March, months after the company and its chief financial officer, Meng Wanzhou, were indicted on a raft of charges, including bank fraud and conspiracy to commit money laundering, in what prosecutors described as a scheme to circumvent U.S. sanctions against Iran.

In their redacted court papers, prosecutors said Cole co-signed a letter in January 2019 to then-Deputy Attorney General Rod Rosenstein in which he stated he was representing Huawei. Later that month, following Wanzhou's arrest in Canada, Justice Department and FBI officials informed Cole that they believed he had a conflict of interest in the case, prosecutors said.

Cole and Huawei had contested the Justice Department's bid to disqualify him from the case, denying that his former Justice Department role created a conflict of interest. In September, Donnelly asked prosecutors to make a nonpublic filing to help her determine whether to disqualify Cole.

Huawei has turned to a stable of major U.S. law firms as it has come under increased scrutiny, hiring lobbyists from Sidley, Jones Day, Steptoe & Johnson LLP and Squire Patton Boggs.

A Sidley team that included Thomas Green, the defense lawyer for former Trump aide Rick Gates, and Mark Hopson, the managing partner of the firm's Washington office, registered in August to lobby for Huawei on "export controls, trade and economic sanctions, and other national security-related topics."

Huawei hired Jones Day and another firm with ties to the Trump administration—Morgan, Lewis & Bockius—to challenge a law funding the Department of Defense that prevents the U.S. government from purchasing the Chinese company's products. The case is pending in the U.S. District Court for the Eastern District of Texas.