A Guyana-born informant aiding the feds in a New York drug probe was gunned down after he left a nightclub in Brooklyn last month, The Post has learned, the New York Post reported.

The murder appeared to be a professional hit, law-enforcement sources said — and the killing could hamper efforts to convince others to become government cooperators.

Burton Dennison, 44, who was gunned down in his Mercedes in East Flatbush, was helping agents on Long Island on “multiple cases” in exchange for reduced jail time on drug charges, said a source familiar with the investigation.

“He was cooperating not only in the case he was arrested on, but also proactively assisting the DEA to ensnare others dealing drugs,” the source said.

“Certainly, any cooperator will have this in their mind when it comes to their decision whether or not to work for ‘Club Fed,’ ” another source said.

And a veteran prosecutor called the murder a “worst-case scenario,” noting that word of the killing would spread “like ripples in a pond” and intimidate informants already wary of cooperating.

“From a law-enforcement perspective, it’s what keeps us up at night,” the source said. “The best thing we can do is come back, full force, on whoever did it.”

Dennison, of Queens, who was married with six kids, was shot once in the head and once in the “lower extremity” on Jan. 13 after leaving the club at 3:30 a.m., according to documents and a source.

He was gunned down a few blocks away at East 46th Street and Glenwood Road.

He started working with the feds when he was busted in September 2011 for selling a kilo of cocaine to an informant — who himself was angling for reduced jail time on drug and money-laundering charges — according to court documents.

He gave up an alleged partner in the scheme when he was arrested, and later agreed to work with the DEA on the Long Island drug cases, sources said. He did not enroll in the federal witness-protection program.

Sources said investigators are probing whether the murder was revenge for his cooperation.

“Mr. Dennison pled guilty pursuant to a cooperation agreement, and following his guilty plea he continued to cooperate with the government,” his lawyer, Eric Franz, wrote to federal Judge John Gleeson. “Unfortunately Mr. Dennison’s worst fears manifested themselves in his being gunned down and murdered.”

Dennison pleaded guilty to conspiring to distribute drugs, and was set to be sentenced Friday in Brooklyn federal court.

The killing of a cooperator in a federal case is unusual.

“It rarely happens,” said Steve Zissou, a defense attorney, even though “cooperating is a very dangerous game.”

Additional reporting by Bruce Golding and Josh Margolin

mmaddux@nypost.com