John Bolton, President Donald Trump's former national security adviser who later became one of his most outspoken critics, is expected to plead guilty in a classified documents case, CNN reported.

Bolton is expected to plead guilty to one count of illegal retention of sensitive national security documents, according to CNN, which cited three sources familiar with the matter. He also agreed to pay a fine of more than $2 million, one source told the network.

A conviction on that count carries a possible sentence of zero to 60 months in prison.

The expected plea deal comes months after federal prosecutors in Maryland charged Bolton over allegations that he kept diary entries from Trump's first White House term at his home.

Prosecutors had accused Bolton of sharing "more than a thousand pages of information about his day-to-day activities" through a personal email account with two unauthorized individuals, whom CNN previously identified as his wife and daughter.

CNN noted that the alleged transmission of classified information is not part of the charge to which Bolton is expected to plead guilty.

Bolton, who served as Trump's national security adviser for about a year during the first Trump administration, had originally faced eight counts of transmission of national defense information and 10 counts of retention of national defense information.

A hearing is scheduled for June 26, according to the court docket.