A gunman’s conviction in the murder of a 2-year-old boy is about to be reviewed by the US Supreme Court — without crucial prosecution evidence that Bronx District Attorney Darcel Clark refused to introduce, The Post has learned.

Clark “withheld consent” for ex-Bronx Assistant District Attorney Adam Oustatcher to submit a 33-page brief to the high court, even though tragic tot David Pacheco Jr.’s mom wanted the paperwork filed, the now-former prosecutor wrote in legal papers.

Clark’s move came despite imprisoned killer Darrell Hemphill granting his consent to the filing and Clark agreeing to let eight groups of outside advocates file submissions on Hemphill’s behalf, Oustatcher wrote.

They include the National Association of Criminal Defense Lawyers, the Innocence Project, the American Civil Liberties Union, the Constitutional Accountability Center and a group of law professors, online court records show.

In a one-sentence ruling ahead of oral arguments set for Tuesday, the Supreme Court last week denied Oustatcher’s motion for consideration of his “friend of the court” brief, without explanation.

In his motion, Oustatcher said that legal papers filed by both Hemphill’s lawyers and Clark’s office “omit extensive and significant portions of [Hemphill’s] trial to such a degree as to not provide a fair and accurate record of the underlying proceedings.”

In his proposed brief, Oustatcher — the lead prosecutor during the 2015 trial — cited evidence of a “carefully orchestrated fraud” led by Hemphill to frame another man, Nicholas Morris, for opening fire following a fistfight near Harrison and West Tremont avenues on Easter Sunday 2006.

Pacheco was struck as his mother, Joanne Sanabria, drove him and a minivan full of kids in matching outfits on their way to church and a family dinner, and the toddler died in her arms in a taxicab as it raced to a nearby hospital.

The charges against Morris led to a 2008 mistrial and a subsequent plea bargain in a case overseen by then-Bronx DA Robert Johnson, who’s now a Bronx Supreme Court Justice.

Oustatcher, who began re-investigating Pacheco’s slaying in 2012, described that prosecution as “horribly botched” and Justice, “a testament to gross professional incompetence marred by questionable eyewitness identifications not subjected to any scrutiny, critical witnesses not interviewed, and a shocking refusal to test crucial DNA evidence.”

In his appeal, Hemphill, 43, contends that his Sixth Amendment right to confront his accuser was violated when prosecutors introduced Morris’ guilty plea as evidence at his trial without calling Morris to the witness stand.

Morris — who admitted possessing a .357 Magnum revolver, and not the 9 mm pistol used to kill — Pacheco — was barred from re-entering the US following a trip to Barbados.

Sanabria asked Oustatcher to argue against Hemphill’s appeal but Clark’s office opposed his intervention in a June 24 email, Oustatcher wrote in his motion.

During a July 16 teleconference, Clark’s deputy general counsel, Gina Mignola, cited a state rule that bars lawyers from representing clients in matters related to work they did earlier for the government, Oustatcher wrote.

But even after Oustatcher said there was no “attorney-client relationship” between him and Pacheco’s mom, Clark’s office “continued to withhold its consent without further explanation,” according to his motion.

In a prepared statement, a Clark spokeswoman said Monday, “We are not commenting on our filings or anything else, as we will be arguing the case before the court tomorrow.”

A court system spokesman also said, “As this is a pending matter, it would be inappropriate for [Johnson] to comment. “

Oustatcher declined to comment and said Sanabria also didn’t want to discuss the case.