Hizbollah is one of Iran's most notorious proxy groups - Reuters

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

The discovery was shocking. Thousands upon thousands of small packages gathered together, each one containing ammonium nitrate - a common component in homemade bombs.

To innocent eyes, the disposable ice packs looked innocuous enough. By folding one, the water pouch inside would burst and mix with the contents, rapidly turning cold.

But in this case the purpose was sinister. By cutting them open, pure ammonium nitrate could be extracted. Mixed with other components and it became fatally explosive.

Britain’s security services were familiar enough with ammonium nitrate. It had been used to kill 168 people in Oklahoma City in 1995 and 202 people in Bali in 2002.

Those bombings had been thousands of miles away. Now here were vast quantities sat on the outskirts of London.

Just as shocking was the timing. The spring and summer of 2015 had been a historic phase for Iran’s relations with the West thanks to the emergence of the nuclear deal.

Britain, the US, Germany, France, Russia, China and the European Union had agreed to lift economic sanctions on Iran, who in turn agreed to stop pursuing nuclear weapons.

In April a framework was agreed. By July a deal was done. Foreign ministers for the signatories appeared together on stage grinning. Iran, it seemed, had taken a step towards peace.

But in the autumn of 2015, a potential British bomb factory had been uncovered which was run by one of Iran’s most infamous proxy groups, Hizbollah.

The Oklahoma city bombing was carried out with the same ingredient Credit: Bob Daemmrich

The militia had emerged in Lebanon in the 1980s with the backing of Iran. As it grew in prominence it continued to receive Iranian financing and support.

Hizbollah’s composition, made up of a political wing with elected representatives in Lebanon and a militant wing, created splits among Western countries deciding how to treat it.

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The entire group had been labelled a terrorist entity by the US in the 1990s. But in Britain, just the militant wing was banned - a designation which came to be seen as favourable to Hizbollah.

The setup had led senior British counter-terrorism figures to believe there was some form of understanding that Hizbollah would not target the UK directly.

But the discovery of the London plot threw the presumptions about Hizbollah - and potentially Iran’s good faith - into doubt. It needed urgent investigation.

To understand what they were facing, agents from MI5 and officers from Metropolitan Police’s Counter Terrorism Command launched a covert operation.

It became clear, according to well-placed sources, that the UK storage 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. Years later a New York Hizbollah member would appear to seek out a foreign ice pack manufacturer.

Why ice packs? They provide the perfect cover, according to sources - seemingly harmless and easy to transport.

Proving beyond doubt they were purchased for terrorism was tricky. But the most relevant case was in Cyprus. Just months earlier, a plot with startling similarities to the one discovered in London had been busted.

Palestinian militants from the Popular Front for the Liberation of Palestine walk next to a poster of Hezbollah leader Sheikh Hassan Nasrallah during an anti-Israel rally Credit: WAEL HAMZEH/EPA-EFE/REX

There, a 26-year-old man called Hussein Bassam Abdallah, a dual Lebanese and Canadian national, was caught caching more than 65,000 ice packs in a basement.

During interrogation he had admitted to being a member of Hizbollah’s military wing, saying he had once been trained to use an AK47 assault rifle.

Abdallah said the 8.2 tonnes of ammonium nitrate stored was for terrorist attacks. He pleaded guilty and was given a six year prison sentence in June 2015.

In Abdallah’s luggage police found two photocopies of a forged British passport. Cypriot police say they did not tip Britain off to the London cell, though some foreign government did.

But they did offer assistance when made aware of the UK case, meeting their British counterparts and sharing reports on what they had uncovered.

MI5's intelligence investigation is understood to have lasted months. The aim was both to disrupt the plot but also get a clearer picture what Hizbollah was up to.

Such investigations can involve everything from eavesdropping on calls to deploying covert sources and trying to turn suspects. The exact methods used in this case are unknown.

Both David Cameron, then the prime minister, and Theresa May, who as home secretary was meeting weekly with the MI5 director general, were briefed on the operation.

Soon conclusions begun 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 itself would have been the target. And the ammonium nitrate remained concealed in its ice packs, rather than removed and mixed - a much more advanced and dangerous state.

On September 30, the Met made their move. Officers used search warrants to raid four properties in North West London - three businesses and one residential address.

That same day a man in his forties was arrested on suspicion of terrorism offences under section 5 of the Terrorism Act 2006. If convicted, a maximum sentence of life in prison was available.

What was discussed when the man was interviewed is not known. Nor is his name or nationality. His was the only arrest - despite sources saying at least two people were involved in the plot. The man was released on bail. Eventually a decision was taken not to bring charges.

The exact reasons why 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 overseas.

Another decision was also taken, this time by UK government leaders - to keep MPs and the public in the dark. The secrecy of the operation must have played a role.

But did Britain's very vocal support for the Iran deal also factor into the equation? Some critics will raise eyebrows.

In the years that followed political debate raged about the UK government's refusal to designate the whole of Hizbollah a terrorist group.

A January 2018 debate in Parliament saw more than a dozen MPs argue for the change. Yet Ben Wallace, the security minister, defended the government's position.

He made no explicit mention of the plot, though there were references to UK efforts to “disrupt those who engage in terrorism”.

At last this February, after mounting pressure from America, the UK proscribed Hizbollah in its entirety - not just its military wing but its political one too.

Finally, more than three years after the London bomb factory plot had been secretly uncovered, it was a criminal offence to support Hizbollah.