As prosecution fails, is time running out for 'fake sheikh'?

His investigations have led to more than 200 convictions and caused acute embarrassment to members of the Royal Family, football managers and assorted celebrities.

Yesterday, however, the undercover reporting methods of the "fake sheikh", Mazher Mahmood, were thrown into question - not for the first time - when three men caught in one of his sting operations were cleared of any wrongdoing at the Old Bailey.

The men from north-west London, Roque Fernandes, 44, Dominic Martins, 45 and Abdurahman Kanyare, 53, denied allegations of plotting to supply terrorists with "red mercury", a rare radioactive material which could be used to make a bomb.

After deliberating for two weeks, the jury returned verdicts of not guilty to charges arising from Mr Mahmood's investigation for the News of the World.

Tipped off by an anonymous source, known as Mr B, Mr Mahmood posed as a potential seller of the deadly chemical and met the three men, accompanied by undercover police officers. The resulting story was splashed on the front of the paper in September 2004.

Before the start of the three-month trial, estimated to have cost more than £1m, the defence team urged the judge to throw out the case, arguing it would be an abuse to let it go ahead.

Mr Mahmood, whose investigations have prompted indiscretions from the former England football manager Sven Goran Eriksson, the Countess of Wessex and Princess Michael of Kent among others, was accused of being an agent provocateur who had trapped the three men.

Stephen Solley QC, representing Mr Martins, said there was a "huge danger of accepting Mr Mahmood's word in respect of any matter". He added that the deal was motivated by, "money on the one hand and selling newspapers on the other". "We submit that justice went out of the window," Mr Solley said.

The News of the World said it was "disappointed" by the outcome of the trial and defended its journalist and his story, insisting it would continue to pursue any investigation with a "clear public interest".

In a statement, the newspaper said: "Our story resulted from a thorough and legitimate investigation by Mazher Mahmood, one of the paper's most senior and experienced reporters whose exposés have led to over 200 convictions. The News of the World involvement in this investigation and subsequent trial, was conducted under the direction of senior anti-terrorist police officers.

"We are entirely satisfied that the methods used in the investigation were not only wholly proper but were both authorised and, from an early stage, continued in close liaison with the police."

Commenting on the case, Roy Greenslade, professor of journalism at City University, London, and a former editor of the Daily Mirror, said: "I don't know if it will stop him in his tracks. I think it should give the News of the World's editor and senior executives pause for thought about the use of subterfuge and of characters like this tipster who played a role in entrapping these men. It crosses a line between reactive journalism and proactive journalism. Either by charm or circumstance, people are lured into doing things they should never think of."

Mr Mahmood recently clashed with the Respect MP George Galloway, who claimed the reporter attempted to trap him by posing as an Islamic businessman. When Mr Galloway responded by circulating a photograph of Mr Mahmood, the News of the World applied for a temporary injunction on the grounds that his life was threatened. The injunction was later lifted.

In another infamous sting in November 2002, Mr Mahmood exposed an alleged plot to kidnap Victoria Beckham and her sons. Five men were arrested, but the case against them collapsed.

Mazher Mahmood's front-page news

George Galloway March 2006

The Respect MP called Mahmood an "agent provocateur" and a "disgrace to journalism" after the reporter invited him to dinner at the Dorchester in March, posing as a Muslim businessman. The News of the World denied that Mahmood tried to "entrap" Mr Galloway and made anti-Semitic comments at the meeting. When Mr Galloway circulated a photograph of Mahmood, the News of the World sought a temporary injunction, which was later lifted.

Victoria Beckham Kidnap Case November 2002

Mahmood uncovered a plot to kidnap Victoria Beckham and her sons and ransom them for £5m. Five men were arrested but the case collapsed after it emerged the main witness, Florim Gashi, had been paid £10,000 for the story.

Freddy Shepherd March 1998

Freddy Shepherd, the chairman of Newcastle United, and Doug Hall, his deputy, made a string of sensational indiscretions to Mahmood in a trawl of lap-dancing clubs in Marbella, on the "Toongate tapes". These included describing Newcastle women as "dogs" and calling Kevin Keegan "Shirley Temple".

John Alford August 1997

The former Grange Hill star John Alford claimed he was the victim of "Dark Age journalism" after he was sentenced to nine months in prison for agreeing to supply Mahmood with cocaine. The actor was sacked from his £50,000-a-year job playing a fire fighter on ITV's London's Burning when the revelations appeared in the News of the World.

The Countess Of Wessex April 2001

The Earl of Wessex's wife met the fake sheikh at the Dorchester in the hope of securing a £20,000-a-month PR contract with a Saudi prince. In the ensuing "Sophie tapes", the Countess of Wessex made a series of comments about members of the Government and claimed that Cherie Blair "hates" the countryside.