GAUNT NCIS superstar Mark Harmon is haunted by a heart attack nightmare — and alarmed pals fear the former UCLA quarterback could be on his last legs!

The once-strapping 6-foot-1 college football star, who ran for nearly 1,000 yards as he plowed through linebackers over two seasons in the ’70s, has lost at least 20 pounds and plunged to a withered 157, a top weight loss expert says.

Now a shadow of his former buff self, the 65-year-old actor, who plays Navy Special Agent Jethro Gibbs, has also turned reclusive and rarely leaves his home, according to sources.

“Mark looks like he’s wasted away!” says a source close to the actor. “And he seems to have a lot less energy.

“When he isn’t working, he spends much of the time holed up inside his house.”

A worried pal notes: “It’s very unusual for him. He’s always been so active and athletic. Friends are deeply concerned!”