A special court hearing the case indicted Mr Khan and former foreign minister Shah Mahmood Qureshi for violating the Official Secrets Act on Monday, Geo TV reported.
Mr Khan, the chair of his Pakistan Tehreek-e-Insaf (PTI), and Mr Qureshi were indicted in the case linked to making the contents of a diplomatic cable public last year for his alleged claims that his ouster from the office was part of a US-led conspiracy.
Mr Khan now faces a sentence ranging from life imprisonment to the death penalty, his defence lawyer Umair Niazi said.
The classified cable was leaked after it was sent from Pakistan’s ambassador in Washington to Islamabad after Mr Khan was ousted in a no-confidence vote in April 2022.
Special court judge Abual Hasnat Zulqarnain rejected the petitions moved by Mr Khan and his deputy’s lawyers.
The next hearing in the case has been scheduled for 27 October and will involve the presentation of witnesses’ statements and cross examination.
Mr Khan, who is Sharif’s successor and main political rival, remains imprisoned in a corruption case and is serving a three-year sentence, further complicating his aspirations to run in the upcoming elections slated for late January.
Mr Khan and Mr Qureshi have denied the allegations, calling the charges politically motivated.
Mr Niazi said he was confident Mr Khan and Mr Qureshi will be acquitted as they had done “nothing wrong”.
The court proceedings took place in a detention facility outside Islamabad at the Adiala jail, where Mr Khan is being held in connection with the secret cable case.
The diplomatic cable, dubbed Cipher, has not been made public by either the government or Mr Khan’s lawyers. But Mr Khan reportedly waved it during a public rally.
Another defense lawyer for Mr Khan, Naeem Panjutha claimed on Monday that the Mr Khan and Mr Qureshi were “indicted in a hurry” with the intention of quickly convicting the popular opposition leader. He said his client is being denied a fair trial.
This marks the second time Mr Khan has been formally charged.
The international cricketer-turned-politician is facing more than 170 cases, including charges ranging from contempt of court to terrorism and inciting violence.
He previously told The Independent that he faces a threat to his life, but insisted on contesting elections. He was disqualified from running for public office five years following his conviction in a corruption case in August.
Later that month, an Islamabad High Court suspended that sentence in a legal victory for Mr Khan.
The former prime minister still enjoys a large following and had galvanised mass support in the days leading to his arrest. He also remains a leading opposition figure.
According to a Gallup survey in June, six out of ten Pakistani citizens still want him as prime minister. These ratings were double than those for Sharif, who had led the country as interim prime minister.