Washington DC - John Bolton, who served as Donald Trump's national security advisor before becoming an outspoken critic of the president, plans to plead guilty to one count of retaining classified documents, US media reported on Thursday.

The 77-year-old veteran diplomat is expected to enter the guilty plea at a hearing in federal district court in Maryland on June 26, CNN and other outlets said.

Bolton, one of several Trump foes to be hit with criminal charges since the Republican president returned to the White House, was indicted in October on 18 counts of transmitting and retaining top secret national defense information.

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CNN said Bolton will plead guilty to one felony count of illegal retention of sensitive national security information, which carries a sentence of up to 60 months in prison. He will also pay a fine of more than $2 million.

Bolton was accused of sharing classified files by email with two "unauthorized individuals" who have not been identified but are believed to be his wife and daughter.

According to the Justice Department, the documents "revealed intelligence about future attacks, foreign adversaries, and foreign policy relations."

Bolton served as Trump's national security advisor in his first term and later angered the administration with the publication of a highly critical book, The Room Where It Happened.

He frequently appears on television news shows and in print to condemn the man he has called "unfit to be president."

Trump has repeatedly lashed out at his former aide and withdrew his security detail shortly after returning to the White House in January of last year.