An off-duty Mount Vernon cop, driving the wrong way on the New England Thruway, was killed in a fiery crash when he slammed head-on into a tractor-trailer in the Bronx early this morning, authorities said.

Reginald Velez, 29, was driving south in the northbound center lane in his Lexus sedan near exit 11 when he plowed into the rig at about 2:35 a.m., officials said. The Lexus immediately burst into flames.

Velez was pronounced dead at the scene. An autopsy and toxicology tests will be conducted.

The truck driver was not injured.