The News of the World has said it is "disappointed" by the outcome of today's "dirty bomb" trial.
Today three men were acquitted of conspiring to buy red mercury, following an undercover News of the World investigation.
In a statement the NoW said: "Our story resulted from a thorough and legitimate investigation by Mazher Mahmood, one of the paper's most senior and experienced reporters whose exposes 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.
"The court heard that Mr Mahmood had 'followed police instructions to the dot' and that he had 'a very experienced' legal mind.
"This newspaper will continue to pursue any investigation which has a clear public interest."
The methods of News of the World investigations editor Mazher Mahmood are likely to come under scrutiny again following the acquittal of three men.
Roque Fernandes, 44, Abdurahman Kanyare, 53, both of Edgware, and Dominic Martins, 45, of Stanmore, were arrested in September 2004 after apparently trying to buy red mercury as part of a News of the World undercover operation.
The court heard that the substance could have been used to create a radioactive bomb which could devastate London.
Mahmood played the part of a Muslim who had more than a kilogram of the substance to sell.
Questions were raised during the trial over what exactly red mercury was and even if it existed.
It is the second time a high-priofile criminal prosecution has failed following a Mahmood investigation.
In June 2003, the prosecution of five men arrested and charged with plotting to kidnap Victoria Beckham collapsed amid controversy about the NoW’s methods.
Mahmood paid one of the men, Florim Gashi, £10,000 to act as an informant.
The trial verdict is likely to be seized upon by MP George Galloway and journalism professor Roy Greenslade who in April called on Mahmood to “retire”.
They spoke out after Galloway was himself targeted by one of Mahmood’s fake sheikh stings – when he says he was enticed to make anti-semetic comments and to solicit illegal political donations.
Greenslade said at the time: “There are clear signs that he [Mahmood] engages not just in subterfuge, which can be employed in the public interest, [but] he engages in the use of agents provocateurs, inciting crimes to occur, which he can then report on.
"I believe he dreams up crimes for other people to commit or for agents provocateurs to make them contemplate. The clearest example was the Victoria Beckham kidnap plot that wasn't. It's clear that Mazher Mahmood needs to be stopped."
Mahmood joined the NoW in 1992 and in a Press Gazette interview in 1997, said that he couldn't remember a week in which he hadn't received a death threat.
For more background on Mazher Mahmood’s colourful career click here.