Forgery equipment and "high quality" fake documents used in terrorism were discovered by police buried in the back garden of his Manchester home and the man - who cannot be named - is facing a retrial overseas on terror charges.
Italian security services bugged the Tunisian's conversations, found him to have "intricate knowledge of terrorism" and are likely to seek his extradition for a second time, The Sunday Telegraph can disclose.
This newspaper reported exclusively last week how Home Secretary Theresa May's order to keep the man out of Britain because he was a threat to national security had been overturned by judges in the Court of Appeal.
He is accused of playing a key role in a Europe-wide terror cell which recruited Islamic extremists to fight jihad in Iraq and Afghanistan, possibly against British and American forces.
Judges also banned naming the Tunisian - identified in legal documents by the initials MK - who was first extradited from Britain to Italy in 2008 to face terror charges but was acquitted last year.
senior officer with the Genoa carabinieri special operations unit, one of Italy's main counter-terrorism teams, said MK was the subject of a three year investigation which involved monitoring telephone calls made from Italy to him in England.
"When he was arrested in Manchester a significant amount of material used for forging documents was found in his house and buried in the garden - the forged documents were of a very high quality," said the marshall, who declined to be named.
"We remain convinced he was in touch with people who had contacts in Iraq and Afghanistan with terrorist cells.
"From my experience of him and the investigation he certainly had an intricate and particular knowledge of terrorism but as I say after four years it is difficult to say now if he was dangerous but at the time he was and so that is why we are appealing.''
And an Italian security source said: "During the investigation we bugged some private and public premises and we are now developing transcripts of those Arabic conversations that were bugged.
"Some of those conversations were bugged in the immigration detention facility during the case in court. The latest ones involve MK and we intend to use them as evidence in our appeal."
He said MK was believed to be a "key figure" in the terror organisation which was under investigation.
"MK was eventually convicted of being with false documentation and a number of rubber stamps, false paperwork and items to make forgeries were found in the garden of the house where he was arrested in England.
"We were disappointed that he was acquitted along with the others but we are in the process of appealing that sentence. I strongly believe that at that moment in time MK was a significant terrorist figure," he said.
Italian prosecutors will allege MK was involved in an extremist group inspired by a secret militant branch of the radical international Muslim group Hizb ut-Tahrir.
MK, who is in his 50s, was arrested in a dawn raid by Greater Manchester Police in November 2007, along with 17 other suspects in Italy and other European countries.
Poisons and ignition devices were seized at addresses in a number of northern Italian cities.
MK was acquitted of terrorist charges in a Milan court in July last year but convicted of falsely procuring a document. Because of time spent on remand he did not have to serve a further sentence and was arrested at London City Airport the following month.
Judges ruled he had a right to stay in Britain to appeal against the Home Secretary's decision.
A preliminary hearing in the Italian prosecutors' appeal was heard last week and a full hearing against MK is due to take place at the Corte d'Assise d'Appello in July.
The operation of the Italian legal system means the court can not only overturn MK's acquittal but also convict him of the charges.
Upon any conviction, Italian authorities will apply for a new warrant to extradite MK to Italy from Britain, where they believe he is currently located, although the Home Office refused to discuss his whereabouts.
The development raises significant questions about the British Court of Appeal's handling of MK's case, and whether the Home Office and British security services have liaised adequately with their Italian counterparts.
The appeal judges' ruling made no mention of a possible retrial in the Italian courts. And because it is not known whether MK is under surveillance by MI5 and Special Branch, it is difficult to predict how easily British authorities could trace him if Italy requests his extradition for a second time.
The first extradition of MK took 12 months to complete after his lawyers brought a lengthy appeal under human rights laws. They claimed his removal to Italy would place him in danger of further extradition to Tunisia where he would be at risk of torture and ill-treatment.
A second extradition request would be likely to see MK's lawyers repeating their arguments under the European Convention on Human Rights.
Last night a Tunisian human rights lawyer said the Jasmine Revolution in January, which led to the ousting of the country's long-time dictator President Zine El Abidine Ben Ali, had cleared the way for MK and others to return to Tunisia.
Samir ben Amor, secretary of the Tunisian Association of Political Prisoners, said all political prisoners had been freed and their convictions set aside.
“This is a different country now. He can come back without any risk,” said Mr Ben Amor. “Hundreds of Tunisians in the same position have already come back from England and elsewhere.”
Mr Ben Amor’s statement significantly undermines MK’s claim to the British courts that he would face ill-treatment or persecution if deported from this country.
The Sunday Telegraph knows the identity of MK but has been prevented from disclosing it by the courts because he is an asylum seeker.
He came to Britain in 2001 and lived in Manchester with his wife and daughters. He had earlier been convicted in his absence of terrorist offences by a military court in Tunisia.
But his claim for asylum is likely to be based on being a member of an Islamist party in Tunisia which was banned before the regime was overthrown by this year's popular uprising - meaning it would have no grounds for success.
A Home Office spokesman said: "We do not comment on individual cases, but public protection is the first duty of government and there are strong measures in place to ensure national security."