The UK and Irish governments are under pressure to set up inquiries into the Omagh bombing after a judge ruled it was potentially plausible the attack could have been prevented.

Mr Justice Horner directly recommended that the UK Government carries out an investigation into alleged security failings in the lead up to the 1998 Real IRA atrocity that killed 29 people, including a woman pregnant with twins.

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

Delivering judgment in a legal challenge against the UK Government’s refusal to hold a public inquiry, the judge said a human rights compliant probe is needed to examine whether a more “proactive” security approach against dissident republican terrorists in the lead-up to the bombing may have thwarted it.

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, Michael Gallagher, whose son Aiden was killed in the blast, launched a judicial review against the UK Government’s refusal to order a public inquiry.

Delivering the conclusions to the long-awaited judgment in Belfast High Court, Mr Justice Horner said: “I am satisfied that certain grounds when considered separately or together give rise to plausible allegations that there was a real prospect of preventing the Omagh bombing.

“These grounds involve, inter alia, the consideration of terrorist activity on both sides of the border by prominent dissident terrorist republicans leading up to the Omagh bomb.

“I am therefore satisfied that the threshold under Article 2 ECHR (European Convention on Human Rights) to require the investigation of those allegations has been reached.”

In 2019, the Government ordered a public inquiry into the events around the 2017 Manchester Arena terror attack.

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 he did not want to be “prescriptive” about the methodology.

He also said he did not have the powers to order the authorities in the Irish Republic to act, but he expressed hope the government in Dublin would take a decision to investigate events around the bomb, which was transported into Northern Ireland from the Irish Republic.

The judge said: “Any investigation will have to look specifically at the issue of whether a more proactive campaign of disruption, especially if co-ordinated north and south of the border, had a real prospect of preventing the Omagh bombing, and whether, without the benefit of hindsight, the potential advantages of taking a much more aggressive approach towards the suspected terrorists outweighed the potential disadvantages inherent in such an approach.”

In a brief hearing, Mr Justice Horner only read the conclusion of his judgment to the court on Friday. He 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.

Mr Gallagher described the ruling as “absolutely amazing”.

He added: “We feel vindicated, this has been a great day for the families.

“We just hope that both the British and Irish governments, as the judge has recommended, will look at these issues and move them forward very quickly.

“We knew from, really, over 20 years, that this was a preventable atrocity, but it’s one thing for me to say it, it’s an entirely different thing for a senior High Court judge to.

“I just felt sadness on one side and relief on the other that, you know, we got it right and people have looked at this and believed us.”

Mr Gallagher said a public inquiry is the only form of investigation that could answer the questions around Omagh.

“We hope today is the beginning of the end, not the end,” he said. “This is a huge step forward for justice and for the Omagh families.”

The UK Government said it will now take time to consider the judgment, while Irish premier Micheal Martin said his government will examine all the options and will do what is “necessary” to uphold citizens’ rights.

Mr Gallagher launched his action against then Northern Ireland secretary Theresa Villiers after she declined to order a public inquiry in 2013.

Ms Villiers argued a probe by former police ombudsman Dr Michael Maguire was the best way to address any outstanding issues.

In the legal case, Mr Gallagher claimed intelligence from British security agents and Royal Ulster Constabulary officers could have been drawn together to prevent the dissident republican bombing.

On August 4 1998, 11 days before the bombing, the RUC received an anonymous telephone call warning there would be an “unspecified” terrorist attack on police in Omagh on August 15.

The force’s Special Branch, which handled intelligence from agents, took limited action on the information and a threat warning was not sent to the sub-divisional commander in Omagh, an investigation by former police ombudsman Baroness Nuala O’Loan found.

An RUC review concluded in 2000 that the information should have been passed to the commander.

Responding to the judgment, current Northern Ireland Secretary Brandon Lewis said: “We recognise that today the court has set out that there are ‘plausible allegations that there was a real prospect of preventing the Omagh bombing’ and that more should be done to investigate this.

“The UK Government will take time to consider the judge’s statement and all its recommendations carefully as we wait for the full judgment to be published.”

Taoiseach Mr Martin said the judgment demands “very serious reflection and analysis” by both the UK and Irish governments.

“We will analyse that judgment and we will do what is necessary in terms of the citizens on the island of Ireland,” he said.