IT'S the nuclear nightmare that actually happened: A 1960s US bomber broke up in mid air, a warhead dropped - and automatically armed itself. And this was over North Carolina.

A declassified document, part of a new book titled Atomic Gaffes by Eric Schlosser, reveals how a defective hydrogen bomb, some 260 times more powerful than the one dropped on Hiroshima, came dramatically close to flattening a large swathe of the US county of Goldsboro on January 23, 1961.

This was just three days after President John F Kennedy had made his inaugural address as President.

The radioactive fallout could have affected millions as it drifted over Washington, Baltimore, Philadelphia and even New York.

Two Mark 39 four-megaton hydrogen bombs were aboard a B-52 bomber which encountered difficulties shortly after taking off from the Seymour Johnson Air Force base in Goldsboro.

The heavy, multi-engine jet went into a tail-spin and broke up in mid-air during a live Cold War deployment.

The two bombs broke free.

One of the free-falling weapons automatically deployed its parachute and armed its trigger mechanism. There were four "fail-safe" devices built into the bomb. Three of them failed.

All that prevented the plummeting super-weapon from going off was a single electronic switch.

Both hydrogen bombs ended up burying themselves deep in fields in the North Carolina countryside.

The document, obtained through a freedom of information investigation, reveals the lie behind persistent US Government denials that American lives have ever been put at risk through safety flaws with its nuclear arsenal.

A senior engineer responsible for the safety of nuclear weapons conceded in a secret 1970s study into the accident: "One simple, dynamo-technology, low voltage switch stood between the United States and a major catastrophe".

This final line of defence could easily have been shorted by a simple electrical spark, he wrote.

The engineer, Parker F Jones, wrote his secret report "Goldsboro Revisited or: How I learned to Mistrust the H-Bomb" some eight years after the accident.

The title was a reference to Stanley Kubrick's 1964 film, Dr Strangelove or: How I Learned to Stop Worrying and Love the Bomb.