More than 20 fraud trials could be declared unsafe after it emerged a star prosecution witness is a fantasist with no qualifications.

Andrew Ager, who has testified in a string of court cases, was called to give evidence in a multi-million pound diamond scam trial that later collapsed.

In total, victims lost £3.5million but the eight defendants walked free from court after a judge was forced to deliver not guilty verdicts on multiple fraud charges.

The men were cleared after it emerged Ager, a witness for the prosecution, had no academic qualifications - and could not even remember if he had passed his A-Levels.

Called to give evidence in the trial, which centred on the 'carbon credit trading market' he admitted he had never read a book on the subject - but had once watched a documentary about it.

City of London Police have 'sincerely apologised' for the case collapsing, and the Crown Prosecution Service say they are reviewing past cases involving Ager.

The Metropolitan Police, who received a complaint from one of the defendant's lawyers, say they will be referring the matter to the National Crime Agency for investigation.

Mr Ager, who is now said to have been removed from NCA's list of approved experts, was quizzed about his background by defence lawyers for Steven Sulley, one of men acquitted of fraud.

Andrew Ager (pictured) was found to not have any academic qualifications and admitted never having read a book on carbon credits, despite being used as an expert witness on the matter

Under questioning he said he kept sensitive material given to him by police in a cupboard under the stairs, and some was lost during a leak.

Judge Nicholas Loraine-Smith was alerted to the fact that Mr Ager did not have any relevant qualifications.

He said that Ager had provided a 'wholly misleading' statement to the police, which included information that was 'cut and paste' from another statement.

'Andrew Ager is not an expert of suitable calibre,' the judge said.

'He had little or no understanding of the duties of an expert. He had received no training and attended no courses. He has no academic qualifications. His work has never been peer-reviewed.'

The court also heard that during a three-hour telephone call, Ager attempted to dissuade the defence's expert witness, Dr Marius Cristian Frunza – who the judge said was 'undoubtedly an expert in this field' – from giving evidence.

The judge said: 'On what I heard it seems likely that Mr Ager was keen that his evidence should not be challenged and he felt threatened by the prospect of a far more impressive expert appearing in an area of expertise in which he hoped to continue to make a living.'

Mr Ager's inadequacy as an expert surfaced during a fraud trial at Southwark Crown Court (pictured) on Wednesday

Mr Sulley's defence team, led by barrister Narita Bahra QC, said Mr Ager had been used in more than 20 fraud prosecutions as an expert witness.

The Metropolitan Police is now referring the matter to the National Crime Agency.

Lawyers estimate the current trial, which heard evidence for three weeks before it was halted, has cost millions in public money.

In a statement posted on the website of law firm 2 Hare Court, the legal team 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.'

What are carbon credits, how do the scams work and what was Ager's role as a prosecution witness? Carbon credits are a global scheme that are aimed at reducing the amount of greenhouse gases released into the atmosphere. The credit is a permit that can be traded for money so a company can emit a certain amount of carbon dioxide. It is bought on the understanding that the release of carbon dioxide is offset by another company elsewhere in the world. It is a complicated area not regulated by the City watchdog, the Financial Conduct Authority and over the past few years scams involving them have been rife. Many investors, who are usually called out of the blue by scammers, have reported they can’t sell or trade their carbon credits and so can’t make any profit. They may be offered carbon credit certificates, voluntary emission reductions, certified emission reductions or an opportunity to invest directly in a ‘green’ scheme or project that generates carbon credits as a return on investment. The scam may claim carbon credits are ‘the new big thing’ in commodity trading, that industries now have to off-set their emissions. Ager's role in the prosecution of these trials was to make the case that the defendants must have known they were asking investors to buy a product that could not be sold on and was effectively worthless. The prosecution case relied on Mr Ager's evidence making the point to the jury that anyone, with any knowledge of the market, would have been fully aware how little prospect there was of being able to make any profit. That would enable the prosecutors to prove that the defendants knew they were committing a fraud. Source: Financial Conduct Authority Advertisement

They claim that City of London Police officers had made no note of their meetings with Mr Ager or any record of the material they had given him.

The defence team also said that the CPS and the police did not disclose the fact that another expert witness, Peter Buckie, had faced misconduct allegations in 2013.

The 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 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.

'Regrettably we haven't been able to provide these victims with the level of service we continually strive for, and for that we sincerely apologise.

'Since this case started we have been changing how we and others in policing investigate fraud.

'Undoubtedly there is key learning we will take from this case which we will use going forward to provide a better service to the victims of fraud.'

The acquitted defendants are: Mr 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 Crown Prosecution Service (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.'

The CPS spokesman added: 'We are considering past cases to identify any in which Andrew Ager appeared as an expert witness and will consider any action necessary once these have been fully reviewed. Mr Ager will not be used as an expert witness in any future cases.'