The prosecution’s star witness against former Trump campaign chairman Paul Manafort admitted on the stand Tuesday that he cheated Manafort, cheated on his wife and may have even cheated the president’s inaugural committee.

Former Manafort business partner Richard Gates copped to embezzling money from Manafort to help fund an expensive extramarital affair 10 years ago.

Manafort’s lawyer suggested that the affair — which included a London pad, first-class plane trips and luxury hotels — cost up to $3 million.

Gates also acknowledged at the federal trial in Alexandria, Va., that he may have been wrongly reimbursed for inflated expenses submitted to President Trump’s then-inaugural committee.

He has pleaded guilty to helping Manafort hide payments from Manafort’s consulting work with pro-Russia politicians in Ukraine to dodge taxes.

Gates testified Tuesday that Manafort hit the roof when faced with an enormous tax bill.

“WTF,” Manafort wrote in a 2015 email, after learning how much he’d have to shell out to Uncle Sam, Gates said.

Gates is cooperating in hopes of a lighter prison sentence.