John Bolton, a former national security adviser to Donald Trump, has pleaded guilty to mishandling classified security information as part of notes he compiled for a book.

Bolton, now a prominent critic of the US president, was indicted on 18 counts related to improper handling of classified material, and initially pleaded not guilty.

On Friday, he admitted to a single charge of illegal retention of classified information. The documents he retained included diary entries containing national defence information, some of it classified at the top secret level.

Bolton faces a prison sentence of up to five years and has agreed to pay $2.25m (£1.7m) in fine, prosecutors said.

Bolton will also debrief national security officials on the classified information he illegally retained as well as perform 100 hours of community service, the BBC's US partner CBS News reported.

After the judge read the allegations against Bolton in court on Friday, including about sending diary entries with sensitive information to his family members, Bolton said the accusations were accurate.

"I did your honor," Bolton said about whether he committed the actions at hand today. He added he was "sorry for it."

He is set to be sentenced on 28 October, US media report.

Bolton was fired from Trump's first administration in 2019. His 2020 memoir, The Room Where It Happened, recounted his time working under Trump, portraying him as a president who was ill-informed about geopolitics.

The White House filed a lawsuit to block publication of the book, arguing that it contained classified information and had not been properly vetted. A judge denied the request and the book was released days later.

The US Department of Justice then opened an investigation into whether Bolton had mishandled classified information by disclosing parts of it in the book.

He was also accused of transmitting some of the classified materials from his time as national security adviser to two relatives.

Bolton has continued to be critical of the president in the time since. Trump, in return, has suggested that Bolton should go to jail and called him a "sleazebag".

The indictment said that at one point a hacker gained access to Bolton's account, where documents were stored and sent an apparent threat to cause "the biggest scandal since Hillary [Clinton]'s emails were leaked".

Bolton's indictment came on the heels of other high-profile criminal cases brought against Trump critics, including former FBI Director James Comey and New York Attorney General Letitia James.