The incident at a base in North Dakota, just south of the Canadian border, is the latest in a series of nuclear mistakes which have plagued the US Air Force in recent months.

In August last year a B-52 bomber was mistakenly armed with six nuclear warheads and flown from Minot Air Force Base, the site of the latest incident, to Barksdale Air Force Base in the southern state of Louisiana.

And in August 2006, four Air Force fusing devices for ballistic missile nuclear warheads were mistakenly shipped to Taiwan.

Defence secretary Robert Gates announced a sweeping shake-up of the Air Force leadership last month, blaming them for failing to deal fully with the series of nuclear-related incidents.

Ike Skelton, chairman of the US House of Representatives' Armed Services Committee, called the latest incident very troubling. "The new Air Force leadership, when confirmed, must take decisive and urgent steps to restore the culture of respect that our strategic weapons deserve and our national security demands," he said.

An investigation by military and National Security Agency experts into the latest incident found that the missile launch codes were outdated and remained secure at all times.

Air Force Colonel Dewey Ford, a spokesman at Peterson Air Force Base in Colorado Springs, Colorado, said: "This was just a procedural violation that we investigated. We determined that there was no compromise."

He said the Minot-based crew had code devices that were no longer usable because new codes had been installed in the missiles. But the lapse on July 12 was serious enough to prompt an investigation by the 91st Missile Wing, in conjunction with codes experts at the 20th Air Force, US Strategic Command and the National Security Agency.

No one has been punished, but a continuing review by base commanders will determine what, if any, actions will be taken against the sleeping crew members.