A plainclothes NYPD cop violated department protocol by using a chokehold while handcuffing a Staten Island man who died following the arrest, Mayor de Blasio and Police Commissioner Bill Bratton said Friday.

That officer and another cop who arrested Eric Garner, 43, were both placed on desk duty following the controversial Thursday arrest, Bratton said at a press conference.

“The two officers who were engaged in the arrest of the deceased – one is an eight year veteran and the other is a four-year veteran – both officers have been assigned to desk duty pending the investigation,” Bratton said.

“There’s a very high chance that the officer who put him in a chokehold would lose his job,” said a law-enforcement source who is familiar with the investigation.

“It violates protocol. It violates a direct rule which is never to put somebody in a chokehold.”

The Staten Island District Attorney and NYPD Internal Affairs are investigating the arrest.

Police said they handcuffed the 350-pound Garner in Tompkinsville at about 3:30 p.m. yesterday because he was selling untaxed cigarettes, but witnesses told the Post that Garner was not peddling smokes and had tried to break up a fight between two people who ran off before cops arrived.

Garner died of a heart attack as an ambulance took him to Richmond University Medical Center, cops said.

Mayor de Blasio said it was too early to jump to conclusions but did say a video of the arrest was disturbing.

“It was very troubling. I watched it the same way a family member would watch it, and it was very sad to watch it,” Hizzoner said.