Christopher Steele, the former British spy who authored the anti-Trump dossier, is refusing to cooperate with John Durham's probe into the origins of the Trump-Russia investigation.

Steele's representatives recently told Durham's team that the former MI6 agent would neither cooperate with the investigation nor speak to Durham, Reuters reported.

Sources told Reuters that Steele believes Durham's investigation is "overly politicized" and that "he would not be treated fairly."

While speaking to students at Oxford University in England on Friday, Steele reiterated his refusal to cooperate with the investigation, claiming that he and his private investigations company, Orbis Business Intelligence, had already "done our duty," the Daily Beast reported.

"As far as I'm concerned, we've said everything we have to say on the matter," Steele said.

The development is significant because Steele is presumably a key witness in the central matters of Durham's investigation. That is because the FBI heavily relied on the dossier that Steele compiled — which was filled with unverified, salacious, and false claims about President Donald Trump — when securing FISA warrants against Carter Page.

In December, DOJ Inspector General Michael Horowitz revealed his investigators had found at least 17 "significant errors or omissions" in the FBI's application for surveillance warrants against Page.

Meanwhile, Steele told the Oxford University students that he stands by the validity of his largely debunked dossier.

"We stand by the integrity of our work and the integrity of our sources," he said. "Trump himself doesn't like intelligence because its ground truth is inconvenient for him."