A special court has indicted the 71-year-old and his former foreign minister, Shah Mahmood Qureshi, according to the Pakistan Tehreek-e-Insaf.

The charge is related to a classified cable sent to Islamabad by Pakistan's ambassador in Washington early last year, which Khan is accused of making public.

At the time, Khan held up the alleged letter, claiming it was proof that he was being threatened and that his ousting was a US conspiracy allegedly executed by the military and the government in Pakistan - claims denied by both Washington and Pakistani officials.

Khan denies making the document public and says its contents appeared in the media from other sources.

His party says the case is due to begin on Friday and that it intends to challenge the charges.

Khan, a former cricketing hero turned politician, was ousted from power following a no-confidence vote in parliament last year.

He then led protests against the government to push for an early general election, and against the military, which he accused of trying to sideline him.

Khan was convicted of corruption earlier this year for unlawfully selling state gifts in office.

But the conviction was suspended in August and his three-year prison sentence overturned.

He was later arrested in connection with other cases, including instigating violence and the official secrets case.

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Khan has had dozens of legal cases filed against him. He has denounced them as an effort to banish him from politics.

A guilty verdict under the Official Secrets Act could bring up to 14 years in prison or even a death sentence, according to Umair Niazi, one of Khan's defence lawyers.

However, he said he is confident Khan and Qureshi, who have both denied the charges, will be acquitted as they have done "nothing wrong".

Khan's party said Monday's hearing took place on the premises of Adyala Jail, near Islamabad, where Khan is being held.

He is disqualified from the upcoming general election because of his corruption conviction, but his legal team is pushing for him to be released on bail before the vote.