Image copyright Met Police/PA Image caption Mazher Mahmood tricked Tulisa Contostavlos into believing he was a film producer

Undercover journalist Mazher Mahmood, known as the "Fake Sheikh", has been found guilty of conspiring to pervert the course of justice.

The reporter, 53, was found to have altered evidence in the collapsed drugs trial of singer Tulisa Contostavlos.

His driver, Alan Smith, 67, was also found guilty of the same charge following a trial at the Old Bailey.

Lawyers say 18 other people targeted by Mahmood now plan to bring civil claims against him, which could total £800m.

Some of the individuals were convicted of crimes which, they argued at the time, came as the result of false evidence.

The Fake Sheikh's most famous stings

Media lawyer Mark Lewis said the civil claims would "dwarf" those brought following the phone-hacking scandal.

"Over the last 25 years, innumerable lives have been ruined by the dishonest actions of Mazher Mahmood," he said. "People have lost their livelihoods, their homes and relationships, with some spending time in prison."

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 investigative journalists did important work "but Mahmood clearly went too far".

"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," he said.

"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."

The 'Fake Sheikh'

The self-styled "king of the sting" claims to have helped in the convictions of 100 criminals during his 25 years as an investigative journalist

He posed as various different characters, including a sheikh

His targets have included spot fixing Pakistani cricketers, ex-England football manager Sven-Goran Eriksson, former Blue Peter presenter Richard Bacon, the Countess of Wessex and Princess Michael of Kent

His methods were controversial and some trials stemming from his investigations collapsed, including that of an alleged attempt to kidnap Victoria Beckham

Many details about him were a mystery, and his contract prevented his photograph ever being published

Mahmood has defended his actions, telling the Leveson Inquiry into press standards that the "ends justify the means"

The court heard that Miss Contostavlos had been targeted by Mahmood, posing as an influential film producer who wanted her to star in a Hollywood blockbuster.

Mahmood - a former News Of The World investigations editor - met the singer at the Metropolitan Hotel in London in 2013 and she 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 from Purley, south London, handed evidence to police.

'Agent provocateur'

But her trial was eventually thrown out, the Old Bailey 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.

As Smith drove Miss Contostavlos 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 him his draft police statement, the singer's anti-drugs comments were removed, the court heard.

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."

Analysis

Image copyright PA

By David Sillito, BBC media correspondent

This is about much more than just the trial of Tulisa Contostavlos.

Mazher Mahmood, known as the Fake Sheikh, was the master of the tabloid undercover sting.

For more than 20 years, his stories have not only made front page news - they have helped send people to prison. This verdict throws new doubt on his tactics and evidence.

Three cases based on his evidence have already been abandoned, another 25 have been referred to the Crown Prosecution Service. Six have been taken up by the Criminal Cases Review Commission, including that of the former London's Burning actor John Alford.

Another 18 people want to take civil legal action against him.

However, there is a limit to what many of his victims can do as libel actions have to be brought within 12 months of a story being published.

After a long career in which he has exposed, embarrassed and in some cases helped convict his victims, the shadowy Mahmood is now headline news himself and his reputation is in shreds.

Image copyright PA Image caption Alan Smith drove Miss Contostavlos home after her meeting with Mahmood

Neither defendant gave evidence during the trial, but it was said on Mahmood's behalf that there had been a "misunderstanding" of his evidence as he was "steamrollered" with complicated questions.

Miss Contostavlos's comments in the car would not have stacked up against the "clear and incontrovertible evidence" against her, the defence claimed.

Mahmood, who also worked for the Sunday Times and Sun on Sunday, has been suspended by News UK since the collapse of the Tulisa trial.

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."

Both men will be sentenced on 21 October.