Ashraf Islam, 30, was arrested May 23 after he walked into a police station and confessed that he had made threats against Prince Harry, The Sunday Telegraph reported.
Islam pleaded guilty to charges of making threats against the prince two days later in Uxbridge Magistrates' Court and is now awaiting sentencing. He faces up to 10 years in prison.
The arrest came just a day after the slaying of soldier Lee Rigby in the Woolwich district of London. At least 10 people have been arrested in that case, all but two of whom have been charged.
Meanwhile, a review of the 28-year-old prince's security detail is under way in fear that Harry may be a target of the Taliban's because of his two tours in Afghanistan as an Apache pilot and his support for military causes, The Mirror reported.
"Prince Harry's security is constantly reviewed and has been massively stepped up in the last 10 days. This case and the horrific murder of Lee Rigby show that there is constant danger. Harry's security team cannot be complacent," a royal official said. "He is a target for many reasons and his protection officers know they have to be extremely vigilant."
"While there is relief this man has been caught, Harry's security team are mindful this may be a sign of things to come," the official said.