Undercover journalist Mazher Mahmood, known as the Fake Sheikh, has been found guilty of conspiring to pervert the course of justice.
Mahmood, 53, was found to have tampered with evidence in the collapsed drugs trial of singer Tulisa Contostavlos.
His driver and co-defendant, Alan Smith, 67, was also found guilty of the same charge following a trial at the Old Bailey.
Prosecutors said Mahmood had had a "vested interest" in her prosecution.
Both men will be sentenced on 21 October.
The court heard that Miss Contostavlos had been targeted by the self-styled "king of the sting", who posed as an influential film producer who wanted the singer to star in a Hollywood blockbuster.
Mahmood met the singer at the Metropolitan Hotel in London and allegedly arranged for him to be sold half an ounce of cocaine by one of her contacts for ?800.
The former N-Dubz star and X Factor judge was later arrested and charged with being concerned in the supply of a class A drug, after Mahmood handed evidence to police.
But her trial was eventually thrown out, the court was told, after driver Smith was found to have changed his police statement, removing comments that she made to him expressing her disapproval of hard drugs.
Referring to Mahmood, prosecutor Sarah Forshaw QC said: "He knew that if it could be shown that he had acted improperly as an agent provocateur, inducing Miss Contostavlos to do something she would not otherwise do, his own credibility and standing and the prospect of conviction in the case might both be severely damaged."
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