Sean Doyle, the former principal scientist at the Government's Forensic Explosives Laboratory, has come forward to claim flaws in the evidence used to convict the failed 21/7 bombers.
He also cast doubt over the work of the prosecution's expert witness, Dr Stuart Black, at the 2007 trial.
In his witness statement, which forms a key part of their appeal, Mr Doyle sets out his "concerns regarding some of the scientific evidence presented by the prosecution".
Tests shown at the trial led to the conclusion that the four men's homemade devices were "potentially viable," despite failing to explode on the day.
The men claimed their targeted attacks on three tube trains and a bus were a hoax.