A failure to act on an informer tip-off or use intelligence and surveillance evidence about previous terror attacks are among the reasons for a judge directing a fresh investigation into the Omagh bombing.

Mr Justice Mark Horner said a new probe should also examine whether a politically motivated "de-escalation" of the security approach to dissident republicans in the months before the 1998 attack resulted in crucial intelligence not being acted upon.

The alleged security failings by the authorities were outlined in the judgment in a legal challenge against the UK Government's refusal to hold a public inquiry into the Real IRA atrocity, which killed 29 people, including a woman pregnant with twins.

In July, Mr Justice Horner delivered his conclusions in the long-running judicial review, ruling it was potentially plausible that the attack could have been prevented.

He directly recommended that the UK Government carry out a human rights-compliant investigation into alleged security failings in the lead-up to the August 1998 attack.

While having no jurisdiction to order the Irish government to act on the matter, the judge also urged authorities there to establish a probe, in light of his findings.

However, at the time of his ruling, Mr Justice Horner did not outline the rationale behind his conclusions.

He set that out today as he delivered the fuller judgment at Belfast High Court.

The judge made clear that his role was not to make definitive conclusions on issues raised by the applicant - bereaved father Michael Gallagher - but rather to assess whether he had established an arguable case that merited examination in a fresh investigation.

Michael Gallagher (left) with his solicitor John Fox at Belfast High Court today

He rejected six of the 10 grounds submitted by Mr Gallagher's legal team but accepted the other four.

Among those was that an RUC informer, known as Kevin Fulton, passed on information to his handlers in the days before the Omagh attack, indicating that the Real IRA was soon to move a bomb across the border into Northern Ireland.

The court case heard claims that the authorities did not act on this tip due to the fact that a Real IRA bomb-maker identified by Mr Fulton was another security force informer.

The judge said Mr Fulton's evidence could not be "dismissed summarily", despite what he described as an attempt by government lawyers to portray him as "irredeemably unreliable".

He said the court should be slow to make a judgment one way or the other on Mr Fulton's reliability.

"I am satisfied that it is arguable that the intelligence supplied by Kevin Fulton, either on its own or more importantly in conjunction with other intelligence about the activities of those who planned and planted the Omagh bomb and other bombs, had a real prospect of preventing this tragedy," he said.

The judge said there was a strong case for taking proactive steps against those engaged in dissident terrorism prior to the bomb.

However, he said there may have been "good reasons" why the authorities adopted a "cautious approach", including the risk of uncovering an informant within the Real IRA.

Mr Justice Horner said the authorities may have also believed that taking a more robust approach against dissident suspects had the potential to destabilise Northern Ireland's fragile peace process.

Another ground upheld by the judge was over failures to act on surveillance data gathered by UK intelligence agency GCHQ around the time of the attack and mobile phone cell site data that, if analysed appropriately, would have linked Omagh suspects to a series of other bombings in Northern Ireland in the months leading up to the atrocity.

The judge said this information could have prompted police action against the suspects.

"There is no doubt that the authorities in Northern Ireland could have made life very uncomfortable indeed for those dissident republicans who could have been identified on the open evidence which was potentially available as being involved in terrorist activities in the six months leading up to Omagh," he said.

Relatives of Omagh bomb victims Stanley McComb (left) and Michael Gallagher outside court today

He said a "proactive" joint approach by the RUC and Garda could have offered a "real prospect of preventing the bomb".

Mr Justice Horner also upheld a ground claiming that police investigating the Omagh bomb were not given all information on suspects held by RUC Special Branch.

"There was arguably a failure of policy - instead of encouraging authorities to use all legal power given to them to deal with terrorism, there was a de-escalation of security which was impaired by political thinking," he said.

The judge said there was a "plausible argument" that there was a "failure to access all the intelligence potentially available in respect of earlier dissident attacks" and that this would have enabled the authorities to disrupt dissidents and that "consequently there was a real prospect of avoiding the Omagh bombing".

The Omagh bomb, which happened months after the signing of the Good Friday peace agreement, was the worst single atrocity of the Northern Ireland conflict.

Eight years ago, Mr Gallagher, whose son Aiden died in the blast, launched the judicial review against the Government.

Mr Justice Horner said he was not going to specifically order that the UK probe into the Omagh bomb takes the form of a public inquiry, explaining that he did not want to be "prescriptive" about the methodology.

He noted that other inquiries had proved costly and had not always delivered outcomes that were satisfactory to those involved.

The judge reiterated that he did not have the powers to order authorities in the Irish Republic to act, but expressed hope that the Dublin government would take a decision to investigate events around the bomb, which was transported across the border into Northern Ireland.

In July, Mr Justice Horner was unable to read the full open judgment setting out his reasoning because the person whose job it was to check the document to ensure it did not contain sensitive material was self-isolating with Covid-19.

Reacting to the judgment, Britain's Northern Ireland Secretary Brandon Lewis said: "My deepest sympathies go out to all those affected by the Omagh bombing.

"It was a great tragedy which resulted in unimaginable suffering for the families of those tragically killed and injured.

"I want again to put on record my sincere regret that the families of those killed and wounded have had to wait so long to find out what happened on that terrible day in 1998.

"We will now carefully consider the full judgment."