Fusion GPS, the opposition research firm behind the infamous “pee dossier” on then-presidential candidate Donald Trump, was first hired by former British intelligence officer Christopher Steele to obtain damaging information on Paul Manafort — weeks before the firm hired him to produce his now-discredited, conspiracy-weaving dossier.

The explosive admission was made by Fusion GSP co-founders Glenn Simpson and Peter Fritsch in the pair’s recently-released book — Crime in Progress — and highlighted in a Monday report by American Greatness.

“Weeks before Trump tapped Manafort to run his campaign, Christopher Steele had hired Fusion for help investigating Manafort,” write Fritsch and Simpson. “The matter had nothing to do with politics and was a typical commercial assignment.”

The Fusion GPS co-founders say their firm cut a nominal agreement to “research Manafort’s finances” for a client of Steele, now infamous for authoring a dossier alleging collusion between President Trump and the Kremlin.

Fritsch and Simpson contend that the retired Secret Intelligence Service MI6 agent’s client was Oleg Deripaska, an oligarch and close ally of Russian President Vladimir Putin. Deripaska sought to determine whether Manafort had stolen money from him, according to the book.

“[Christopher Steele] did not disclose the ultimate client for the job, and Fusion didn’t press the issue,” claim the Fusion GSP co-founders.

American Greatness notes of the Fusion GPS-Steele deal: “That means Glenn Simpson represented not one but two Russian tycoons in 2016: At the same time Fusion was helping Steele on behalf of Deripaska, Simpson also was representing Prevezon, another company owned by a Putin-tied Russian also under investigation by the Justice Department for money laundering.”