The trial of undercover journalist accused of conspiring to pervert the course of justice in the case of singer Tulisa Contostavlos will open today.

Mazher Mahmood, 52, who was known as the "Fake Sheikh", allegedly misled the court during the drugs trial of the former X Factor judge.

The former News Of The World journalist will be on trial at the Old Bailey with Alan Smith, 66, a retired driver from Norfolk.

The popstar who lives in Friern Barnet, was charged in December 2013, with "being concerned in the supply of Class A drugs" and appeared at Westminster Magistrates' Court where she denied being involved.

The case was dismissed when the judge believed prosecution witness Mazher Mahmood had lied to the court.

Mahmood and Smith are charged with conspiring together to do an act, namely that Smith would change a draft statement to police, with the intention to pervert the course of justice, between June 22 and July 22 in 2014.

Prosecutor Sarah Forshaw QC today said Mahmood had a “vested interest” in her prosecution, which put his journalistic reputation on the line.

He had posed as a film producer who wanted the aspiring actress to star as a "bad girl" in a Hollywood blockbuster alongside Leonardo DiCaprio.

Mahmood handed evidence to police which led to Miss Contostavlos being arrested and charged with being concerned in the supply of a class A drug, the court was told.

The reporter was to be called as a prosecution witness in the trial and was called to give evidence in a pre-trial hearing about his methods during the sting.

Miss Forshaw said: “In effect, the hearing in June 2014 put Mr Mahmood and his journalistic process on trial.

“He liked to call himself the king of sting, he boasted in a book he had written of the number of convictions that he personally was responsible for.

“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.”

Miss Forshaw said Miss Contostavlos had expressed her “disapproval of hard drugs to his own driver, that was the bit of the statement that was altered.”

She added: “Mr Smith had told the police officer making the statement that he remembered that while driving Miss Contostavlos and her two friends home from a meeting with Mr Mahmood that she had spoken about someone in her family being dependent on cocaine.

"Mr Smith told the officer that Miss Contostavlos had seemed really negative about cocaine and expressed her disapproval of drugs."

A day after making the statement Smith told police he wanted to retract the part about the singer's negative attitude to cocaine, the court was told. In the intervening 24 hours he had allegedly sent a copy of his interview to Mahmood and they had exchanged a number of texts and calls.

During a pre-trial hearing at Southwark Crown Court, Mahmood said on oath that he had not spoken to Smith about Miss Contostavlos's comments.

Both men deny the charges and the trial continues.