Freddie Scappaticci, the man widely named as the British army's IRA informer 'Stakeknife', has pleaded guilty to two charges of possessing extreme pornographic images.
Bedfordshire Police said Scappaticci, 72, faced two counts under the Criminal Justice and Immigration Act 2008.
He appeared at Westminster Magistrates' Court today.
Scappaticci has always strongly denied claims that he was 'Stakeknife', a high-ranking military mole in the IRA's internal security unit that interrogated and murdered suspected spies during The Troubles.
The court heard the charges related to at least 329 images, including some involving animals. There were no images involving children.
Magistrate Emma Arbuthnot sentenced Scappaticci to three months in custody, suspended for 12 months.
She said: "You have not been before the court for 50 years - and that's good character in my book."
Defending barrister Richard Whittam said the offending took place over 13 days, using 20 search terms.
He said the pornography comprised still images - not moving images - and they were not saved on to his computer.