The safety of convictions secured in more than 20 fraud trials may have to be examined after an expert witness was found to be inadequately qualified.

The Crown Prosecution Service (CPS) has abandoned a case against eight defendants in a carbon credit and diamond sales hearing after defence lawyers cross-examined Andrew Ager.

According to the lawyers’ website, at 2 Hare Court chambers in London, Ager was found to have no academic qualifications and could not remember if he had passed his A-levels.

He also admitted he had never read a book on carbon credits, although he had once watched a documentary about such permits.

The problem emerged when Narita Bahra QC, representing Steven Sulley, one of the eight men acquitted of fraud at Southwark crown court on Wednesday, questioned Ager about his background.

Ager said he kept sensitive material given to him by police in a cupboard under the stairs and that some was lost during a leak. Thereafter it was kept in a locked box on his balcony.

Ager was used in more than 20 fraud prosecutions as an expert witness, she said. “As a result of the cross-examination of Mr Ager, the safety of the convictions in every previous carbon credits prosecution is now in question.”

Her chambers’ statement went on: “It is apparent that there are systemic failures within the investigation and disclosure processes at City of London police that are likely to impact upon all prosecutions undertaken in the last eight years.”

Prosecutors offered no evidence in the trial of the eight men for conspiracy to defraud at Southwark crown court and all the defendants were acquitted.

The case centred on the sale of investment opportunities in carbon credits between November 2011 and February 2014.

Carbon credits are permits that allow companies or countries to emit a certain amount of carbon dioxide gas. They can be traded for cash if the limit is not reached.

City of London police apologised in the wake of the case collapsing. A spokeswoman said: “This has been an enormously complex case of seven years’ duration. The case illustrates the significant change in the way fraud itself has evolved, along with our response in dealing with it.

“Together with our colleagues in the CPS, we apologise that the evidential and procedural issues in the case have led to its dismissal.”

The acquitted defendants were: Sulley, 33, from Orpington, Kent; Christopher Woolcott, 37, from Longfield, Kent; Christopher Chapman, 31, from Fulham, south-west London; Lewis Deakin, 28, from Gravesend, Kent; Marcus Allen, 31, from Brixton, south London; Ashley Hunte, 37, from Beckenham, Kent; Daniel Martin, 42, from West Malling, Kent; and David Pierce, 32, from Billericay, Essex.

A spokesman for the CPS said: “We have a duty to continuously review all our cases. Information has recently been brought to the attention of CPS which has led to the conclusion that our legal test for prosecution was no longer met and that it would be wrong to continue with the case against the defendants.”