By Michael Holden

LONDON (Reuters) - Mazher Mahmood, one of Britain's best known undercover reporters renowned for his "fake sheikh" sting operations, was facing jail on Wednesday after being convicted of plotting to alter evidence in a high-profile court case where he was the main witness.

Mahmood, whose elaborate disguises have duped criminals, celebrities, sporting figures and even royalty, conspired to alter a police statement during the drugs prosecution of Tulisa Contostavlos, a singer and former judge of the British version of the "X Factor" TV talent show.

Contostavlos was set to go on trial accused of supplying cocaine for Mahmood who was posing as an influential Indian film producer and later wrote up the story as an exclusive for the Sun on Sunday newspaper.

Prosecutors said Mahmood, 53, had conspired with his driver Alan Smith to alter a statement given to police in which Smith said Contostavlos had spoken out against drugs on an occasion when he drove her home, making her conviction less likely.

London's Old Bailey court was told Mahmood had a vested interest in her being found guilty as his own integrity was at stake. The case against Contostavlos collapsed in July 2014 after questions arose about Smith's evidence.

Both men, who denied any plot but declined to give evidence at their trial, were found guilty of conspiracy to pervert the course of justice and will be sentenced at a later date.

"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," said Simon Ringrose from the Crown Prosecution Service.

Mahmood is the latest journalist to have been convicted of committing a crime while working for media mogul Rupert Murdoch's British papers following the 2014 conviction of senior staff at the defunct News of the World tabloid for hacking the voicemails on mobile phones.

Mahmood, who always featured in papers with his face obscured, made his name at the News of the World and built a formidable reputation for his stings, once claiming his investigations had led to 253 successful prosecutions.

He carried out the inquiry which led to the 2011 conviction of three Pakistani cricketers for taking bribes to fix incidents in a match against England.

In his most famous exclusive in 2001, he posed as an Arab sheikh to dupe Sophie, Countess of Wessex, who is married to Queen Elizabeth's youngest son Prince Edward, into making indiscreet comments about other royals and senior politicians.

Rebekah Brooks, the chief executive of News Corp.'s British newspaper arm News UK who was charged over phone hacking and cleared, told her trial Mahmood was an incredibly professional journalist whose subterfuge was always in the public interest.

But his stings have also led to other high-profile prosecutions which have collapsed, including the case against five men accused of plotting to kidnap singer-turned-designer Victoria Beckham in 2002 after it was revealed the main witness had been paid by the News of the World.

A spokesman for News UK said they were disappointed by the conviction but had no further comment.

After Wednesday's verdict, media lawyer Mark Lewis said he had been instructed by 18 individuals caught by Mahmood's stings to take civil action against him.

(editing by Stephen Addison)