Criminal convictions in 25 cases where evidence was given by "Fake Sheikh" Mazher Mahmood are being re-examined, the Crown Prosecution Service said.
The CPS said it had also offered no evidence in three live cases where the undercover journalist was a prosecution witness after reviewing the prosecutions in the wake of the collapse of the trial of former X-Factor judge Tulisa Contostavlos in July.
A CPS spokesman said: "We are now considering past cases which resulted in a conviction in criminal courts in England and Wales based on evidence provided by Mr Mahmood, and have identified 25 cases.
"As part of this process, over the coming weeks, CPS Areas will be contacting representatives of the defendants – or defendants themselves as necessary – convicted in these cases in order to provide them with a disclosure pack – details of material which they may consider undermines the conviction in a specific case."
Mahmood, a former reporter for the News of the World and The Sunday Times, was suspended by The Sun on Sunday after the collapse of Contostavlos's trial.
The former N-Dubz star went on trial after allegedly boasting that she could "sort out" cocaine for Mahmood and put the reporter in touch with her rapper friend Mike GLC – real name Michael Coombs.
But both were cleared after Judge Alistair McCreath said there were "strong grounds" to believe Mahmood lied in the witness box and "had been manipulating the evidence".
The CPS spokesman said today: "Following the halting of the trial of Ms Contostavlos and Mr Coombs, we took steps to identify current and past cases involving Mr Mahmood as a prosecution witness.
"We made it our immediate priority to carefully look into live prosecutions in accordance with the Code for Crown Prosecutors and any past cases which involved a defendant still in custody.
"There were no concluded cases where a defendant was still in custody, but we identified three live cases. Each case was looked at individually and no evidence was offered as we concluded that there was no longer a realistic prospect of a conviction."
In September, several trials at Southwark Crown Court in London were halted because they relied on evidence from the undercover reporter.
The CPS offered no evidence against Dr Majeed Ridha and pharmacist Murtaza Gulamhusein, who were accused of illegally supplying an abortion drug by Mahmood for The Sunday Times.
And fixer to the stars Leon "Starino" Anderson and co-defendant Ashley Gordon, 21, had drugs charges dismissed after prosecutors said Mahmood was not reliable.
In terms of cases that resulted in convictions, the CPS said it was providing information to defence teams to "allow them to consider whether there is any basis for their client's case to be referred to the Court of Appeal or to the Criminal Case Review Commission (CCRC)".
Disclosure packs are also being sent to the Law Society, CCRC "and relevant regulatory bodies that we are aware of and who have had cases involving Mr Mahmood", it said.