Terrorists linked to Iran were caught stockpiling tonnes of explosive materials on the outskirts of London in a secret British bomb factory, the Daily Telegraph can reveal.
Radicals linked to Hizbollah, the Lebanese militant group, stashed thousands of disposable ice packs containing ammonium nitrate - a common ingredient in homemade bombs.
The plot was uncovered by MI5 and Scotland Yard in the autumn of 2015, just months after the UK signed up to the Iran nuclear deal.
Three tonnes of ammonium nitrate were discovered - more than were used in the Oklahoma City bombing in 1995 that killed 168 people and damaged hundreds of buildings.
Police raided four properties in north-west London - three businesses and a home - and a man in his 40s was arrested on suspicion of plotting terrorism.
The man was eventually released without charge. Well-placed sources said the plot had been disrupted by a covert intelligence operation rather than in seeking a prosecution.
The discovery was so serious that David Cameron and Theresa May, then the prime minister and home secretary, were personally briefed on what had been found.
Yet for years it was kept hidden from the public - including MPs who were debating whether to fully ban Hizbollah - until today.
It raises questions about whether senior UK government figures chose not to reveal the plot in part because they were invested in keeping the Iran nuclear deal afloat.
The disclosure follows a three-month investigation by the Daily Telegraph in which more than 30 current and former officials in Britain, America and Cyprus were approached and court documents were obtained.
A well-placed source described the plot as "proper organised terrorism", while another said enough explosive materials were stored to do "a lot of damage".
Ben Wallace, the Security Minister, said: "The Security Service and police work tirelessly to keep the public safe from a host of national security threats. Necessarily, their efforts and success will often go unseen."
The Telegraph understands the discovery followed a tip-off from a foreign government. To understand what they were facing, MI5 agents and officers from the Metropolitan Police's Counter Terrorism Command launched a covert operation.
It became clear, according to well-placed sources, that the stockpile was not in isolation but part of an international Hizbollah plot to lay the groundwork for future attacks.
The group had been caught storing ice packs in Thailand, and in 2017, a New York member appeared to seek out a foreign ice pack manufacturer.
Ice packs, seemingly harmless and easy to transport, provided the perfect cover, according to sources. Proving beyond doubt that they were bought for terrorism was tricky. The most relevant case was in Cyprus, where a startlingly similar plot had been busted just months earlier, when Hussein Bassam Abdallah was caught with more than 65,000 ice packs in a basement.
The 26-year-old, who had dual Lebanese and Canadian nationality, admitted to being a member of Hizbollah's military wing and said the 8.2 tonnes of ammonium nitrate were for terrorist attacks. He pleaded guilty and was given a six-year prison sentence in June 2015.
In his luggage police found two photocopies of a forged British passport. Cypriot police say they were not the foreign government agency that tipped Britain off to the London cell.
MI5's investigation is understood to have lasted months. The aim was to disrupt the plot and also get a clearer picture of what Hizbollah was up to.
Soon conclusions began to emerge. The plot was at an early stage. It amounted to pre-planning. No target had been selected and no attack was imminent.
Well-placed sources said there was no evidence Britain would have been the target, and the ammonium nitrate remained concealed in its ice packs, rather than mixed into a more dangerous state.
On September 30, the Met used search warrants to raid four properties and arrested a man on suspicion of terrorism offences. Neither his name nor his nationality have been disclosed.
His was the only arrest, although sources said at least two people were involved.
The exact reasons why he did not face charges remain unclear, but it is understood investigators were confident they had disrupted the plot and gained useful information about Hizbollah's activities in Britain and abroad.
A UK intelligence source said: "MI5 worked independently and closely with international partners to disrupt the threat of malign intent from Iran and its proxies in the UK."
The decision not to inform the public, despite a major debate with the US about the success of the Iran nuclear deal, will raise eyebrows.
Keeping MPs in the dark amid a debate about proscribing Hizbollah will also be questioned. The US labelled the entire group a terrorist organisation in the Nineties, but in Britain, only its armed wing was banned.
The set-up led senior British counter-terrorism figures to believe there was some form of understanding that Hizbollah would not target the UK directly. It was only added to the banned terrorist group list in its entirety in February - more than three years after the plot was uncovered.