The government has been accused of  a "massive cover-up” about the threat from Hezbollah, after it was revealed British security services caught the organisation stockpiling “tonnes” of explosive materials in North West London in 2015.

In an undercover operation almost four years ago, Mi5 and the Metropolitan Police discovered three metric tonnes of ammonium nitrate, the Daily Telegraph has reported.

Then Prime Minister David Cameron and Home Secretary Theresa May were made aware of the operation at the time. North West London is home to the largest Jewish community in Europe.

But, despite this, the government took another three and a half years to proscribe Hezbollah in its entirety, despite parliamentary and Jewish communal organisations repeatedly urging them to do so.

In a letter from Joan Ryan MP, chair of Labour Friends of Israel, to Home Secretary Sajid Javid said she was “horrified” to read about Hezbollah stockpiling explosives in London, and that “without the bravery of our security forces this would have been used to target and murder Jews around the world.”

She added: “We now need to know urgently why the Government failed to disclose this vital information to the public and parliamentarians and resisted for more than three years my calls to ban Hezbollah in its entirety."

The stockpiled ammonium nitrate, which is understood to have not yet been in its mixed and far more dangerous form, was registered at around 3,000 kilograms, 700kg more than the amount of ammonium nitrate used in the 1995 Oklahoma City bombing.

One source told the Telegraph that the plot was “proper organised terrorism”.

Israel’s Kan public broadcaster reported an official said on Monday that Mossad, Israel’s foreign intelligence agency, which had notified the UK about the attack.

Ms Ryan noted that although parliament had subsequently debated “the nature and seriousness of the threat posed by Hezbollah both to the UK and to our allies abroad”, information about the operation was not revealed by the government, which, she said “continue[d] to oppose the full proscription for a further three and a half years.”

Although the military wing of Hezbollah was proscribed as a terror organisation by the UK government in 2001, its political wing was not included, despite many noting that there was no significant separation between different branches of the organisation.

The government finally acted to proscribe Hezbollah in its entirety in February of this year.

Ms Ryan, who left the Labour party in February, also asked whether the Opposition had been made aware by the Government of the plot, noting that the party “failed to support proscription of Hezbollah” during a debate on the subject in February.

Labour leader Jeremy Corbyn once infamously referred to "friends" within Hezbollah.

In a 2002 speech, Hezbollah leader Hassan Nasrallah said: “If they [the Jews] all gather in Israel, it will save us the trouble of going after them worldwide.”

The organisation, widely regarded as a proxy for the Iranian regime, is believed to have carried out attacks against Jews internationally, including in Argentina in 1994 and Bulgaria in 2012.

For years, Hezbollah flags were flown at the annual Al Quds Day rally held in central London until this year's parade, which took place after the full ban came into effect, making it illegal to fly the flag.