Fake Sheikh Mazher Mahmood is facing jail for tampering with evidence in the collapsed drugs trial of pop star Tulisa Contostavlos.
Following a two-week trial at the Old Bailey, a jury found the 53-year-old "King of the Sting" and his driver, Alan Smith, 67, guilty of plotting to pervert the course of justice.
The pair conspired to suppress evidence in the N-Dubz star's trial, which was thrown out at Southwark Crown Court in July 2014.
The singer had been accused of arranging for Mahmood to be sold £800 of cocaine by one of her contacts following an elaborate sting for the Sun on Sunday in May 2013.
During a meeting at the Metropolitan Hotel in London, Mahmood posed as a film producer and plied Miss Contostavlos with alcohol as they discussed an acting role alongside Hollywood star Leonardo DiCaprio.
As Smith drove the former X Factor judge home to Hertfordshire, she allegedly spoke about a family member who had a drugs problem.
When he was interviewed by police about the journey more than a year later, Smith, of Dereham, Norfolk, recalled the conversation.
But a day later, after speaking to Mahmood and emailing his draft statement, the singer's anti-drugs comments were removed, the court heard.
At a pre-trial hearing, Mahmood denied being an "agent provocateur" or that he discussed the drugs conversation with Smith.
But when he was questioned at length in the trial, Mahmood appeared to concede he had talked to Smith about what Miss Contostavlos said about drugs in the car.
Neither defendant gave evidence but it was said on Mahmood's behalf that there had been a "misunderstanding" of his evidence as he was "steamrollered" with multi-faceted questions.
And Miss Contostavlos's comments in the car would not have stacked up against the "clear and incontrovertible evidence" against her, it was claimed.
Defence lawyer John Kelsey-Fry QC told jurors: "Mr Mahmood is not a policeman. He is a journalist.
"Whilst the prosecution may say he boasts of the number of convictions resulting from his work, securing convictions is not actually his job."
He said Mahmood's whole investigation was about exposing the pop star's private face "smoking weed" and "arranging cocaine for mates" set against her public persona as a "role model".
Smith's lawyer, Trevor Burke QC, challenged jurors to try to remember, as his client had done, conversations and events a year earlier.
After her case collapsed in July 2014, Miss Constavlos claimed to reporters she had been the victim of ''a horrific and disgusting entrapment''.
For more than 25 years, Mahmood has enjoyed a position as "King of the Sting" at the now defunct News of the World, Sunday Times and Sun on Sunday with Smith as his "right-hand man".
Mahmood, from Purley, south London, has been suspended by News UK since the collapse of the Tulisa trial.
He claims to have helped put more than 100 criminals behind bars and risked his life on a daily basis to lift the lid on the murky world of crime.
Paedophiles, arms dealers and drug dealers have all found themselves at the centre of his stories, as have celebrities and public figures, including the Countess of Wessex, who was taped calling the Queen ''the old dear'', and Sven-Goran Eriksson, who revealed his plans to quit as England head coach.
The Crown Prosecution Service has since dropped a number of live criminal cases in which Mahmood was due to be a witness and announced a review of 25 past convictions.
The Criminal Cases Review Commission is currently reviewing six cases involving celebrities who were convicted following involvement with Mahmood.
Neither defendant reacted as the guilty verdicts were delivered.
The prosecution asked for costs to be awarded totalling £37,929.
Judge Gerald Gordon adjourned sentencing until October 21 and allowed the defendants continued bail.
As he was leaving court, Mahmood declined to comment to journalists or say whether he would be launching an appeal.
Ben Rose, Miss Contostavlos' defence lawyer, said: "The real scandal in this case is that Mahmood was allowed to operate as a wholly unregulated police force, 'investigating' crimes without the safeguards which apply to the police.
"It was obvious from the outset that Tulisa should never have had to go to court.
"If Mahmood's evidence had been properly stress-tested instead of accepted wholesale by the CPS, we are confident it would have come to the same conclusion.
"Investigative journalists do important work, but Mahmood clearly went too far.
"That he and his driver have now been convicted of conspiracy to pervert the course of justice will hopefully deter other journalists from using entrapment to drive celebrity gossip stories.
"Mahmood's actions brought his profession into disrepute and ruined hundreds of lives in pursuit of better circulation figures.
"The Crown Prosecution Service should not be so credulous in future."
A News UK spokesman said: "We are disappointed by the news that Mazher Mahmood has been convicted. We do not have further comment at this time."
Mark Lewis, media lawyer and partner at Seddons solicitors, has been instructed by 18 people to pursue civil claims against Mahmood which could top £800 million.
Some of the individuals were convicted of crimes which, they argued at the time, came as the result of false evidence.
The claims for "substantial compensation" include people convicted more than 20 years ago.
Mr Lewis said: "Over the last 25 years, innumerable lives have been ruined by the dishonest actions of Mazher Mahmood. People have lost their livelihoods, their homes and relationships, with some spending time in prison.
"Following today's verdict, there will be a significant number of civil claims made against Mazher Mahmood. We anticipate the total sums involved could easily reach £800 million, with some awards dwarfing those seen in the phone-hacking scandal."
Simon Ringrose, specialist prosecutor in the CPS's Special Crime Division, said: "Mr Mahmood portrayed himself as the master of subterfuge and as the 'King of the Sting', but on this occasion it is he and Mr Smith who have been exposed.
"Mahmood and Smith tampered with a statement and then attempted to cover their tracks through lies and the destruction of evidence.
"By piecing together the various strands in this matter, the CPS was able to present a compelling case to the jury."