BBC
Mazher Mahmood.
Rupert Murdoch's News
Mark Lewis, a media lawyer at Seddons, said in a statement that he has been instructed by 18 people to pursue civil claims against Mahmood, the so-called "Fake Sheikh."
Most of Mahmood's stings for the Sun on Sunday and News of the World centred around narcotics. He would disguise his identity - often posing as a wealthy Arab - and induce a well-known individual to buy a quantity of drugs. He would then publish the story and often his victims were prosecuted, and some convicted.
Some of the claims date back more than 20 years. Seddon said the individuals involved believe that they were caught up in Mahmood's stings as the result of false evidence.
"Over the last 25 years, innumerable lives have been ruined by the dishonest actions of Mazher Mahmood. People have lost their livelihoods, their homes and relationships, with some spending time in prison," Lewis said.
"There will be a significant number of civil claims made against Mazher Mahmood. We anticipate the total sums involved could easily reach £800 million, with some awards dwarfing those seen in the phone hacking scandal."
The Guardian reported that Lewis is acting on behalf of clients including former model Emma Morgan and John Alford, a star of ITV drama "London's Burning." The newspaper said American financier John Bryan is also pursuing legal action against News Corp.
Mahmood convicted of perverting the course of justice.
A News UK spokeswoman declined to comment on the legal claims. She said: "We are disappointed by the news that Mazher Mahmood has been convicted. We do not have further comment at this time."
Mahmood used his undercover stings to entrap British pop star Tulisa Contostavlos in 2013. Mahmood posed as a film producer and promised her a Hollywood career. During the sting he asked Contostavlos to help him secure £800 ($1,000) worth of cocaine, which the singer did.
The Sun on Sunday published the story and Contostavlos was charged. The case went to trial in 2014 but swiftly collapsed after the judge said there was "strong grounds to believe" that Mahmood "lied" at a pre-trial hearing.
On Wednesday, Mahmood was found guilty of perverting the course of justice for tampering with evidence ahead of the Contostavlos trial. A court was told that he conspired to change a witness statement from his driver, Alan Smith.
Smith originally told police that while driving Contostavlos for Mahmood's sting, the singer expressed her disapproval of hard drugs because a relative was an addict. But Smith changed his statement the following day to remove the anti-drugs comment. The prosecution alleged that he had done this in consultation with Mahmood.
The journalist will be sentenced on 21 October and could face prison.