Manhattan federal prosecutors Wednesday showered praise on Mayor Bill de Blasio donor-turned-cooperator Jona Rechnitz, telling a judge that the businessman deserved the utmost leniency at his upcoming sentencing.

“Since making the determination to cooperate fully with the Government in the spring of 2016, Rechnitz has been, without exaggeration, one of the single most important and prolific white-collar cooperating witnesses in the recent history of the Southern District of New York,” prosecutors wrote in a letter to Judge Alvin Hellerstein.

“By every metric by which one measures a cooperating witness, Rechnitz has been both extraordinary and exemplary.”

Rechnitz testified against former jail union head Norman Seabrook, claiming he lavished donations on de Blasio in exchange for access to City Hall, and said he bribed high-ranking NYPD brass in order to get help with arrests and expedite tricky gun licenses.

His testimony led to the convictions of Seabrook and Rechnitz’s old pal Jeremy Reichberg, while ex-NYPD Deputy Inspector James Grant was acquitted.

Rechnitz faces up to 20 years behind bars at sentencing, but prosecutors begged the judge to show mercy.

“The Government respectfully submits that Rechnitz’s cooperation has been of a caliber such that, if genuine leniency is ever available, it should be available in this case,” they wrote.

Rechnitz’s sentencing is set for Nov. 1.