Terrorists linked to Iran were caught stockpiling tonnes of explosive materials on the outskirts of London in a secret British bomb factory, The 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 the Metropolitan Police in the autumn of 2015, just months after the UK signed up to the Iran nuclear deal. Three metric tonnes of ammonium nitrate was discovered - more than was used in the Oklahoma City bombing 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 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.