Barack Obama's confidential passport file has been secretly looked at by three US government officials in recent months, the Bush administration said yesterday, with two of the workers involved fired.

A spokesman for Mr Obama, the Democratic presidential candidate, demanded a full investigation to discover exactly who looked at the Illinois senator's passport file and why.

"This is an outrageous breach of security and privacy, even from an administration that has shown little regard for either over the last eight years," said Bill Burton, a senior Obama spokesman, in a statement.

"Our government's duty is to protect the private information of the American people, not use it for political purposes."

Sean McCormack, the US State Department spokesman, said that Mr Obama's passport file had been accessed by three different contract workers since January.

The contractors accessed information in the file in an unauthorised way, he said.

Two contractors were fired and one was disciplined by the contractor's company, Mr McCormack said. He said the contractors were not linked.

The breach seems like "imprudent curiosity" among the contract workers, said McCormack.

“That is our initial take on the matter,” McCormack said, but added: “We are not being dismissive of any other possibility.”

He added that senior management at the State Department was not aware of the incidents until Thursday afternoon. He said the breaches occurred on January 9, February 21 and March 14.

The contract workers used their authorised computer network access to look up files within the department's consular affairs section, which processes and stores passport information, and read Mr Obama's passport application and other records, in violation of department privacy rules, Mr McCormack said.

He said Condoleezza Rice, the US Secretary of State, was notified on Thursday and that she said the fact that the breach had been detected was proof that privacy safeguards were effective.

A similar data breach took place in 1992 when State Department officials looked up data on Bill Clinton, then a presidential candidate, in an attempt to find out information from the late 1960s, amid unfounded political campaign rumours that Mr Clinton had sought to renounce his citizenship to dodge the draft during the Vietnam War while a Rhodes Scholar at Oxford. They also tried and failed to find evidence that he secretly travelled to communist countries.

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