A British businessman pleaded guilty in a US court on Thursday to attempting to smuggle a key component of the Hawk air defense missile to Iran, officials said.

Christopher Tappin, 66, faces 33 months in jail after reversing his not guilty plea at a federal court hearing in El Paso, Texas.

"Mr Tappin stated under oath that he was guilty of aiding the attempted export to Iran of sensitive military equipment," United States Attorney Robert Pitman said in a statement.

"In so doing, the defendant put at risk the national security of the United States and its allies by trying to sell to Iran the batteries that make the Hawk missiles operational."

Tappin admitted that he "knowingly aided and abetted others" -- including his Cyprus-based business associate Robert Frederick Gibson, 57, and Robert Caldwell of Oregon -- in an illegal attempt to export the batteries to Iran from December 2005 to January 2007.

Tappin's plea closes the book on the case after Caldwell was sentenced to 20 months in prison and Gibson was jailed for 24 months in 2007 for their roles in the scheme.

They were caught by an undercover agent as they tried to export the batteries via Britain without a license. The batteries are on a list of military equipment which requires special export permits.