Former British spy Christopher Steele described FBI Director James Comey’s October 2016 letter to Congress about Hillary Clinton's emails as "unconscionable. Treasonous, really,” according to a new book.

At the time, Steele, 55, a former MI6 officer, was working for opposition research company Fusion GPS to investigate connections between Donald Trump, then the 2016 GOP presidential nominee. At the same time, he was a confidential informant for the FBI. His fury over Comey's intervention, which he believed damaged Clinton while concealing what the FBI knew about Trump, prompted him to push for what became known as the "Steele dossier" in the public domain.

Fusion GPS co-founders Glenn Simpson and Peter Fritsch recount in their new book Crime in Progress: Inside the Steele Dossier and the Fusion GPS Investigation of Donald Trump that they and Steele, founder of the Orbis Business Intelligence company, were livid that Comey wasn’t making public pronouncements about ithe FBI's Trump-Russia investigation.

Steele responded to Comey’s October letter by publicizing his Trump-Russia allegations, speaking to left-wing journalist David Corn days before the Nov. 8, 2016 election for an article that was headlined: "A Veteran Spy Has Given the FBI Information Alleging a Russian Operation to Cultivate Donald Trump."

The article led the FBI to jettison Steele as an informant. The Fusion GPS authors say Steele told his FBI contact that “any misstep by Orbis or Fusion pales in comparison to what Comey did in disclosing the Hillary investigation.”

Comey announced the FBI wasn’t recommending criminal charges related to former Secretary of State Hillary Clinton’s private email server in July 2016, but criticized Clinton for “extremely careless” handling of classified information. The FBI investigation was reopened when, in late September 2016, thousands of emails belonging to Clinton aide Huma Abedin were found on the laptop belonging to her husband, disgraced former New York Rep. Anthony Weiner. Comey sent a letter to Congress on Oct. 28, 2016, saying the FBI uncovered emails possibly pertinent to the Clinton investigation.

According to the book: "Unconscionable,' Steele said. 'Treasonous, really.' He was so fed up he didn’t bother to call [his FBI handler] in Rome to ask him what was going on."

On Nov. 6, two days before the election, Comey told Congress that the FBI had not changed its decision not to charge Clinton.

The authors state that Comey was guilty of having done "nothing to warn the American public of the FBI’s deep concerns regarding possible illicit dealings between the government of the Russian Federation and the campaign of the Republican nominee for president of the United States,” adding: “In fact, the FBI went out of its way to do the opposite.”

Simpson and Fritsch conclude: “It really did feel like a conspiracy, with a hidden hand guiding Trump into the presidency.”