The feds used a mob rat as a “spy” to secretly pry information out of a reputed gangster’s defense lawyer so prosecutors could use it to prepare for an upcoming racketeering trial, according to court papers filed Sunday.

The unidentified stoolie allegedly called his longtime lawyer, who’s also representing defendant Albert “Al Muscles” Armetta, “to learn confidential information” regarding the planned defense of Armetta and nine cronies busted last year.

Court papers say the turncoat wiseguy then turned over a “slew of information” he extracted from attorney Vincent Martinelli, including “information about other cooperators” and Martinelli’s “opinion on the strength of the case, as well as the opinions and preferred strategies of other co-defendants.”

“In other words, the government input a ‘spy in the camp’ of the defense and, thereby, violated their Sixth Amendment right to counsel as well as other constitutional rights designed to ensure that they receive a fair trial,” wrote lawyer John Meringolo, who represents reputed Bonanno capo and consigliere John “Porky” Zancocchio.

The Manhattan federal court filing calls the situation “virtually unprecedented” and says it’s grounds to throw out the indictment against Zancocchio and five of his co-defendants.

They face trial next month on allegations that include extortion, loansharking, drug dealing and conspiracy to commit murder.

In a heavily redacted court filing late Sunday night, prosecutors called the motion “frivolous” and “filled with factual misstatements,” saying FBI records show they “did not send [the cooperator] to obtain information from [Martinelli] or any other lawyer in this case.”

Martinelli, who didn’t join in the defense motion, declined to comment.