He went on to say that when a president orders a surveillance operation, it will usually leave a paper trail of some form.

He then claimed that in Mr Obama's case, he may have gone to a British intelligence agency to seek external help, so as not to leave any evidence that a wiretap took place.

"Three intelligence sources have informed Fox News that President Obama went outside the chain of command," he said. "He didn't use the NSA, he didn't use the CIA, he didn't use the FBI, and he didn't use the Department of Justice."

How does he know this?

Mr Napolitano is an experienced legal affairs expert who began his career back in 1975. Several of his rulings as a superior judge are considered influential.

However, as a media commentator, he has no official links whatsoever to US intelligence services. He can only provide his own personal opinions on legal affairs.

Though he referred to three "intelligence sources" during the controversial segment of the programme, it is unclear who these sources were.

His claims are therefore closer to being "hearsay" than concrete evidence that could be used to trigger a formal investigation into the wiretapping allegations.