The British and Irish governments will today come under the strongest pressure yet to agree to an international inquiry into the Omagh bombing after claims the attack could have been prevented.
The 1998 car bomb was the worst single atrocity of the Troubles killing 29 people including a woman pregnant with twins, but it is now claimed intelligence officials were recording the bombers' mobile phone calls as they carried out the attack.
Bereaved relatives said their demands for a public inquiry must now be answered after a new documentary claimed the phone-tap information was never passed to detectives probing the bombing which was carried out by the Real IRA.
The British government is also being challenged to reveal if it knew the bombers' phones were tapped, after details of a HomeOffice meeting reportedly showed senior officials discussing how the failure to use information from a telephone intercept led to an unspecified terrorist act.
The bombers have never been brought to justice and Michael Gallagher, whose son Aiden (21) died in the blast, said the case for a public inquiry was now overwhelming.
"We have been demanding a public inquiry since 2002 into the abysmal failure of the police inquiries," he said. "In all conscience the government can no longer resist this."
The new allegations are contained in a BBC 'Panorama' documentary to be screened tonight.
It is unclear if the intelligence officials were listening to the bombers' calls as they made them, or if the calls were recorded, but it is claimed that the full details of the intercepts were not passed to officers investigating the mass murder.
"'Panorama' has established that the (British) Government Communications Head Quarters (GCHQ) was recording mobile phone conversations between some of the bombers as they drove from the Republic to the market town of Omagh," said the programme-makers.
"The revelation that the intelligence services were listening to bombers -- both on the day of the bombing and in the weeks leading up to it -- raises new questions about whether the single worst atrocity of the troubles could have been prevented."
'Panorama' reports what it describes as several well-placed sources saying that GCHQ was monitoring the bombers' mobile phone calls at the request of the Northern Ireland police special branch.
Ray White, a former assistant chief constable in the Police Service of Northern Ireland (PSNI) is said to have confirmed this, although the head of the then Royal Ulster Constabulary, Sir Ronnie Flanagan, is said to have been unaware of the intelligence.
The documentary makers said: "One source tells 'Panorama' GCHQ sent details of the conversations to Northern Ireland within six hours of the bombing.
"But White tells 'Panorama' that his former colleagues in special branch categorically deny this.
"He says they received nothing from GCHQ until the Tuesday after the bombing.
"White also says that special branch was expecting GCHQ to be monitoring the bombers 'in real time' -- so that if it was apparent a bombing was under way they could launch a pre-arranged plan to arrest them.
"When the branch asked GCHQ why they passed nothing over for three days, White reports that GCHQ told them: 'We missed it'. Again, precisely what 'we missed it' means is not clear."
' Omagh: What The Police Were Never Told ' -- BBC One, 8.30pm