The Secret Service, announcing Hill’s death at his home in California, praised the ex-agent’s “unwavering dedication and exceptional service” to the Kennedy family and four other presidents.

Hill was a member of first lady Jacqueline Kennedy’s protective detail in Dallas, Texas, on Nov 22, 1963, when the president was shot by Lee Harvey Oswald while riding in an open car.

Hill, who was in a car just behind the president’s limo, dove onto the back of Kennedy’s vehicle in an attempt to shield him and his wife.

“If I had reacted just a little bit quicker,” Hill later recounted in an interview with the CBS news show “60 Minutes”.

“I’ll live with that to my grave.”

The tragic moment was memorialised in a news agency photograph which showed Hill clambering up the trunk of the president’s vehicle.

He shielded the bodies of Kennedy and his wife during the ride to the hospital, where the president was declared dead.

Hill retired from the Secret Service when he was 43 and wrote several books, including “Five Days in November” about the Kennedy assassination, an event he said had been “seared into my mind and soul”.

“In the blink of an eye, everything changed. Those days remain the defining period of my life,” he wrote.

Another book, “Five Presidents”, recounted his service protecting presidents Dwight Eisenhower, Kennedy, Lyndon Johnson, Richard Nixon and Gerald Ford.