Radicals linked to the Iran-backed Hezbollah movement were found to be stockpiling bomb-making ingredients in London in 2015 in a case that was kept "hidden from the public", the Daily Telegraph reported on Monday.
Following a tip-off from a foreign government, British police and the MI5 intelligence service discovered thousands of disposable ice packs containing three tonnes of ammonium nitrate, the report said, citing security sources.
One man was arrested in a series of four raids in northwest London but he was later released without charge after what the paper said was a "covert intelligence operation" that was not aimed at seeking criminal prosecution.
The paper said that the decision not to inform the public about the discovery, which came shortly after the Iran nuclear deal was concluded, would "raise eyebrows".
The paper said then prime minister David Cameron and interior minister Theresa May were informed but MPs who were debating whether to ban Hezbollah in Britain were not informed.
The paper said similar discoveries of ice packs used to store explosives were made in other parts of the world.
It said ice packs were used as they looked harmless and were easier to transport. The report added that no attack was imminent and the ammonium nitrate had not been weaponised.
Hezbollah's militant wing was banned at the time but the Lebanese Shiite group in its entirety was only added to Britain's terrorist group list earlier this year.