A former principal scientist at the Government's forensic explosives laboratory says he raised concerns before the trial about the reliability of expert findings used by the prosecution.

Channel 4 News has learned the 21/7 bombers are about to launch an appeal alleging a cover up over the scientific evidence at their trial.

A former principal scientist at the Government's forensic explosives laboratory says he raised concerns before the trial about the reliability of expert findings used by the prosecution.

The Crown claimed the devices were viable bombs but sources have told this programme that the new revelations undermine the case.

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The four would-be bombers were convicted in 2007 of conspiracy to murder.

On 9 July 2007, four defendants, Muktar Ibrahim, 29, Yassin Omar, 26, Ramzi Mohammed, 25, and Hussain Osman, 28, were found guilty of conspiracy to murder.

The four attempted bombers were each sentenced to a minimum of 40 years' imprisonment.

The men were always regarded as potential suicide bombers who a fortnight after the 7th of July bombings had emabrked on attempts to maim and murder still further.

It came during a period when London was in the grip of widespread fear.

The men carried their home made devices in their rucksacks and boarded public transport, targetting three different tube trains and a London bus.

When the devices were triggered there was no explosion just a bubbling mixture of flour and hydrogen peroxide.

At trial they claimed it was all just a hoax designed to spread fear and not harm.

The Prosecution insisted the devices were potentially viable and they showed experiments to the court to support the claim. But a former scientist at the Government's forensic explosives laboratory has now come forward to cast doubt on this evidence.

Indeed Sean Doyle was principal scientist in charge of a team at the lab there.

In a witness statement obtained by Channel 4 News, which forms a key part of the appeal submissions, Sean Doyle says he wants to "set out my concerns regarding some of the scientific evidence presented by the Prosecution" during the trial in 2007.

His witness statement said he was: "Specifically concerned about evidence derived from the application of a technique known as Isotope Ratio Mass Spectrometry".

Isotope Ratio Mass Spectrometry this was one of the methods used to determine the viability of the devices.

I have been told that the papers will be lodged in the Court of Appeal tomorrow. They will claim the convictions were unsafe. While many may find it uncomfortable, they raise serious questions about the scientific evidence which helped put the four behind bars.

And the appeal goes further than questioning the validity of the science behind their convictions. It alleges that doubts were expressed at the time and kept hidden and not disclosed at the trial.

Mr Doyle said in his witness statement: "I also expressed my concern verbally to the Metropolitan Police that the methodology employed by (a scientist) and the interpretation of the results may not be reliable. I was asked by (a police officer) about (the scientist's) reliability. I replied along the lines that I was not convinced."

Mr Doyle goes onto say he'd expected that view to be reported to the police officer's colleagues at the Metropolitan Police.

Sources close to the defence have told Channel 4 News that this amounts to a cover up.

Tonight, a CPS spokesperson told Channel 4 News:"We take our disclosure obligations very seriously and will be looking into this as soon as we receive details of the grounds of appeal from the defence."

And the Metropolitan Police said: "We have not seen any papers relating to this application, therefore it would be inappropriate to comment at this stage."

It is not just the Court of Appeals which is being asked to change history. The Criminal Cases Review Commission is also being invited to investigate these claims.

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