British law enforcement officials say they have made their biggest ever breakthrough against organised crime after hacking into an encrypted communications system used to plan drug deals and murder plots.

Since April, more than £50m in cash has been seized in the UK and 746 suspects have been arrested – some of them described as “untouchables” who thought they would never be caught – as part of Operation Venetic. Several suspected corrupt police and law enforcement officials have also been identified.

The National Crime Agency (NCA) said it had mitigated 200 threats to life as part of the operation, thwarting gruesome plans to torture and murder people.

The NCA unveiled the operation after French law enforcement hacked into the end-to-end encrypted service EncroChat this year. Police insist the service was legally hacked, but lawyers for suspects are expected to claim the material was obtained unlawfully when cases come to court.

EncroChat was a “command and control” system for crime barons, and breaking into it triggered hundreds of operations across Europe. Jannine van den Berg, the chief constable of the Dutch police’s central unit, said: “It was as if we were sitting at the table where criminals were chatting.”

For the past three months law enforcement has been picking off drug dealers and firearms importers, with criminals unaware the communications platform was being watched.

Investigators continue to go through a treasure trove of incriminating evidence in the hope that Operation Venetic turns into something few in law enforcement expected they would ever see – a decisive and enduring blow against organised crime and reversal of the record drug supply into and across the UK.

The NCA said EncroChat was used exclusively by criminals, with 10,000 users in the UK paying £1,500 for a six-month contract for handsets. The service included features such as messages that would self-destruct after a certain time and the ability to wipe a device’s data with the input of a code.

The NCA said hacking EncroChat was the law enforcement equivalent of breaking the Nazis’ second world war Enigma code and that dozens of organised crime groups had been dismantled in the UK alone, out of an estimated 4,500.

Every UK police force was involved in hunting down suspects.

Since April, Operation Venetic has led to the seizure of at least £54m in cash alleged to be linked to criminals, along with 77 firearms, including an AK-47 assault rifle, Skorpion submachine guns, handguns, four grenades and more than 1,800 rounds of ammunition.

More than two tonnes of class A and B drugs had been recovered and at least 28m pills of the sedative Etizolam were seized from an illegal manufacturing laboratory that has been linked to deaths on Britain’s streets.

More than 50 expensive cars have been seized, as well as 73 luxury watches and even a barge.

The Metropolitan police said some of those arrested were serious criminals who operated without being known to them. One raid led to the largest single cash seizure – £5m – since Scotland Yard was founded in 1829.

The Met said one crime group hit was operating across the UK, Europe and in the UAE, importing and distributing firearms and supplying drugs to street-level gangs across London, leading to savage attacks on the capital’s streets.

Thousands of handsets have been monitored and millions of messages analysed. Those caught are alleged drug importers and distributors, middle-tier players in the British narcotics trade. Some of the messages recovered may give vital insights into the top tier, such as Colombian cartels responsible for most of the cocaine in the US and Europe.

The NCA said that by 13 June, criminals were aware EncroChat had been broken into by law enforcement and that they were being watched. The NCA said one sent a message saying: “This year the police are winning.”

Those running EncroChat sent a message to users reading: “You are advised to power off and physically dispose your device immediately.”

The NCA’s director of investigations, Nikki Holland, said: “The infiltration of this command and control communication platform for the UK’s criminal marketplace is like having an inside person in every top organised crime group in the country. This is the broadest and deepest ever UK operation into serious organised crime.”

Some of the criminals appear to be successful, otherwise respectable businesspeople. Holland said: “Together we’ve protected the public by arresting middle-tier criminals and the kingpins, the so-called iconic untouchables who have evaded law enforcement for years, and now we have the evidence to prosecute them.”

A gang allegedly attempting to smuggle cocaine from South America on a cruise liner were arrested on 25 April in the Solent area, as well as a crew member of the ship.

The Met commissioner, Cressida Dick, said: “This operation is the most significant activity, certainly in my career, we have ever carried out against serious and organised criminality across London.

“Organised crime groups have used encrypted communications to enable their offending. They have openly discussed plots to murder, launder money, deal drugs and sell firearms capable of causing atrocious scenes in our communities. They were brazen and thought they were beyond the reach of the law.”

The NCA has faced questions about its effectiveness and role. Its leadership is delighted by the successes, which come as they press the government for more resources.

The NCA said two law enforcement officers had been arrested and one had been interviewed under caution. One member of staff from Cheshire constabulary was arrested on 12 June over allegations of misconduct in public office. Two members of the public were also arrested on suspicion of aiding and abetting. They have all been released under investigation.