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Singer Tulisa Contostavlos Image Credit: REUTERS

The drug trial of a former British X Factor judge collapsed on Monday after the judge said the star prosecution witness, an undercover reporter known as the “Fake Shaikh,” had lied under oath.

The Rupert Murdoch-owned Sun on Sunday tabloid reported last year it had caught Tulisa Contostavlos acting as go-between in a deal to sell cocaine to a reporter posing as a film producer.

The scoop was the work of Mazher Mahmoud, a journalist famed for pulling off undercover stings while disguised as a wealthy Arab businessman.

Contostavlos was charged with supplying drugs, but judge Alistair McCreath stopped her trial on Monday, saying there were “strong grounds to believe” Mahmoud had lied in court.

The judge said a chauffeur named Alan Smith had been expected to testify that Contostavlos had told him she disapproved of drugs — “but after [a] conversation with Mr Mahmoud, he had changed his mind.” At a pre-trial hearing, Mahmoud denied speaking to Smith about the issue.

McCreath formally acquitted Contostavlos, who had pleaded not guilty, and co-accused Michael Coombs, also known as rapper Mike GLC.

Contostavlos, 26, gained fame as part of London music group N-Dubz before a stint as a judge on TV talent show The X Factor.

Outside court, Contostavlos accused Mahmoud of “horrific and disgusting entrapment.” She said she had believed she was auditioning for a role in a major movie, with Leonardo DiCaprio as a potential co-star.

“Mahmoud got me and my team completely intoxicated and persuaded me to act the part of a bad, rough, ghetto girl,” she said. “They recorded this and produced this as evidence when I thought it was an audition.”

The Sun said Mahmoud had been suspended pending an internal investigation.

Mahmoud became famous — and feared — for celeb-trapping stings during a long career at the now-defunct News of the World. His targets have included ex-England football coach Sven Goran Eriksson, Princess Michael of Kent and Prince Edward’s wife, the Countess of Wessex.

The reporter, who gave evidence at the trial from behind a screen to protect his identity, claims to have helped put more than 100 criminals behind bars. But some of his exposes have ended in disarray. In 2002 Mahmoud claimed to have foiled a plot to kidnap former Spice Girl Victoria Beckham. Five men were charged, but the trial collapsed when it became clear Mahmoud’s main informant had been paid $18,500 and could not be considered a reliable witness.

The informant, Florim Gashi, testified that the kidnap “plot” had been invented by Mahmoud.

Questions about Mahmoud’s methods long tainted the News of the World — but it was the revelation that others at the tabloid had illegally hacked phones to get scoops that led Murdoch to shut down the newspaper in 2011.