Sergei Skripal had been warned. He had been a double agent, and in 2010, when the Russians released him and others from jail in a spy-swap for ten Russians captured in America, Vladimir Putin made his intentions brutally clear: 'Traitors will kick the bucket,' he announced on state TV.

'Trust me. These people betrayed their friends, their brothers-in-arms. Whatever they got in exchange for it, those 30 pieces of silver they were given, they will choke on them.'

Eight years later, in March 2018, former Soviet military intelligence officer Skripal and his daughter Yulia were fighting for their lives after being poisoned by a deadly Russian nerve agent, Novichok, which had been smeared on the door handle of his home in Salisbury.

It was only the expertise of Government chemical weapons experts at Porton Down a few miles away that saved their lives.

This brazen attempt to perpetrate a state-sponsored execution in an English cathedral city provoked worldwide condemnation. The British Government felt it had no option but to act. The then Prime Minister Theresa May announced on the floor of the Commons that it was 'highly likely' that Putin was responsible and threw out 23 Russian diplomats from the London embassy. In total, more than 150 Russians were expelled from Western countries.

Right: Alexander Litvinenko, defected from Russian security. Poisoned with radioactive polonium in his tea. Left: Boris Berezovsky, financed international opposition to Putin

Such decisive action was welcome, but it was more than a decade too late. The truth is that Putin has been using deadly force to wipe out his enemies from the first days of his presidency in 1999, and nowhere has he done so with greater impunity than in the United Kingdom.

Even before the attempt to murder the Skripals, my team at the BuzzFeed News website had uncovered chilling evidence linking Russia to 14 deaths in this country. But in every single case, the British authorities had shut down the investigations and carried on courting the Kremlin.

Today, the evidence against Putin and his gangster friends continues to mount as Russian agents perpetrate a wider campaign of targeted killings around the world. These murders are deliberate, systematic and personally backed by a president determined to suppress all knowledge of his personal connections to organised crime and his suspected participation in terrorist acts – blamed on Chechen rebels – which have killed hundreds of his own citizens.

James Le Mesurier: Exposed carnage caused by Russian air strikes. Found dead after 'fall from top-floor balcony'

When 48-year-old James Le Mesurier OBE, the British founder of the White Helmets humanitarian group, fell to his death from an upper storey at his Istanbul home last month, many were left fearing that Putin had eliminated yet another opponent.

The White Helmets, a civilian search-and-rescue group operating in Syria, has been shining an unwelcome light on the devastation caused by Russian air strikes. And Le Mesurier had only recently been accused by Moscow of being a British spy and was taking medication for what his wife described as the intense stress brought about by continuous Kremlin smears.

The Turkish authorities are treating the death as suicide. Le Mesurier, as we shall see, is far from the only figure to have crossed the Russian president and then fallen from a great height.

There are good grounds for suspicion. We have established that Putin has poured resources into a laboratory outside Moscow where armies of government scientists have been working for years to refine nerve agents, deadly germs, obscure carcinogens and radioactive poisons designed to kill assassination targets without leaving a trace.

Russia promised to destroy its 40,000-ton arsenal of chemical and biological weapons in 1993 when it signed up to the Chemical Weapons Convention, but the Kremlin has long been cheating on that promise. Attempts to use the chemicals in battlefield conditions were abandoned after it turned out that airborne chemicals could be blown off course by the weather, making them hopelessly imprecise as weapons of war.

But Western spies studying the Moscow poison factory watched in horror as Putin's scientists moved towards developing something else instead – a suite of chemical, biological and nuclear weapons that could wipe out selected individuals.

The use of elaborate poisons to liquidate enemies of the state had long been a staple of the Russian playbook. The Soviet-era KGB had led the world in the art and science of murder, and its weapons labs churned out such deadly marvels as plague sprays, cyanide bullets, lipstick pistols, and ricin-tipped umbrellas. But the new generation of chemicals and diseases developed by Putin's government is more sophisticated by far.

These substances can be administered orally, in sprays, or in vapours to trigger fast-acting cancers, heart attacks, or multiple organ failure. They allow the specialist hit squads from the FSB, the KGB's successor, to eliminate a target while making it look as if the victim had simply succumbed to a sudden illness or died a natural death.

Putin even has an arsenal of psychotropic drugs with which to destabilise his enemies – powerful mood-altering substances designed to plunge targets into enough mental anguish to take their own lives or to make staged suicides look believable.

Why, then, have Britain and the West turned a blind eye while dissenting politicians, journalists, campaigners, defectors, investigators and critics are gunned down, poisoned, hit by cars, thrown out of windows, beaten to death, blown up or thrown into seemingly incurable depressions?

Horrific fall that sparked probe into a string of suspicious deaths It was the terrible fate of Scot Young, a Scottish financier, property magnate and associate of Boris Berezovsky, that persuaded us to investigate this pattern of suspicious deaths. Young was impaled on railings after falling in near-impossible circumstances from an upper- floor window of the apartment in Marylebone, Central London, that he shared with his American girlfriend Noelle Reno. His daughters were astounded at the idea he should have attempted suicide, having spoken to him on the phone just a short while before. Over the course of two years, our team connected Young to a web of deaths in the UK – and one in the US – all of which had glaring links to Russia. We obtained hundreds of boxes of documents, hours of surveillance footage and audio recordings, a huge cache of digital files from forensically restored mobile phones and computers, bags of discarded police evidence and readouts of multiple secret US intelligence files. We tracked down and interviewed more than 200 people, while also gathering information from over 40 current and former intelligence and law-enforcement sources on both sides of the Atlantic. Reporting this story was, at times, a risky business. A man in a black car appeared every night for months outside one reporter's house, another came home to find personal items had been moved around in his bedroom, and it appeared one team member was being followed. Advertisement

Put simply, it has been the cost of doing business with an economically renascent nuclear power that had a stranglehold on Europe's energy supply and a super-wealthy class of oligarchs pouring billions into Western economies, particularly that of London and England's South East.

Successive Western leaders were lulled into the belief that Putin was a man they could do business with – a figure who, with the right coaxing, might finally come in from the cold and integrate the world's largest country into the warmth of the rules-based liberal world order. It was a catastrophic misjudgment.

Putin never really wanted to join the club. He remained what he had always been: a creature of the totalitarian Soviet security state. To his mind, the collapse of the USSR was 'the greatest geopolitical catastrophe of the 20th Century' and he blamed it on the West. It was an outrage to be avenged.

So, having risen through the ranks of the KGB, he arrived at the Kremlin ready to use all the tactics in the Soviet toolkit to restore Russia to its former glory.

While the leaders of the United States and Europe courted him with summits and state visits, handing him the presidency of the G8 and establishing the Nato-Russia Council to foster closer military and political relations, Putin was smiling for the camera, shaking hands, and plotting a silent war on the liberal institutions and alliances upon which the stability of the West depends.

He restored the fearsome power of the Soviet state security apparatus, enriching and empowering the FSB. Anyone who betrayed the motherland, anyone who threatened the absolute power of the Russian state and anyone who knew too much put themselves squarely in the Kremlin's crosshairs.

And every dead body sent a signal. If you cross Vladimir Vladimirovich Putin, there is no safe place for you on Earth.

By 2006, he was sufficiently emboldened to pass new laws explicitly giving the FSB a licence to kill Russia's enemies on foreign soil. Since then, his regime's critics, opponents and traitors have dropped dead in violent or perplexing circumstances in both the United States and Europe – nowhere more than in London, home of an influential group of dissident oligarchs and defected former Russian agents who have made no secret for their hatred of the autocratic Russian leader. They thought they were safe here. They were wrong and paid with their lives.

Some of the names are familiar, particularly Alexander Litvinenko, a defector from the Russian security services who died a horrific death after being poisoned with radioactive polonium in a Central London hotel. Many of the victims, including Litvinenko, had links to Boris Berezovsky, the billionaire mathematician who was once a member of Boris Yeltsin's government.

Berezovsky became an implacable opponent of Putin and made himself the Kremlin's No 1 enemy by financing an international campaign of opposition from his new base in the English countryside – as well as financing investigations into Putin's links to a spate of terrorist attacks against his own people.

One by one, in the years that followed, the lawyers, fixers, dissidents, investigators and businessmen in Berezovsky's circle dropped dead in strange or suspicious circumstances. One by one, the British authorities closed the cases with no investigation – to the astonishment of their US counterparts – and carried on courting the Kremlin.

Left: Gareth Williams, a British spy probing Russians. Decomposing body found locked in a sports bag. Right: Alexander Perepilichnyy, who blew the whistle on a massive Kremlin fraud

Berezovsky himself was found hanging from a shower rail in his Surrey mansion after his security guard had gone to run errands. Like many of Putin's targets, he had been complaining to those around him that the balance of his mind had been altered, as if by a chemical.

The British Government also disregarded explosive intelligence connecting Russian assassins to a string of other deaths on home soil, including that of Gareth Williams, the British spy whose naked body was found decomposing in a padlocked sports bag in his bathtub, and Alexander Perepilichnyy, the Russian financier who dropped dead while out jogging in Weybridge, Surrey, after blowing the whistle on a massive Kremlin fraud.

Other potential victims have received little or no attention. Take Robert Workman, who was blasted to death in January 2004. The 83-year-old, an Army lieutenant colonel in the Second World War, was killed at point-blank range with a sawn-off shotgun as he answered his front door.

At first, police were baffled. Then Hertfordshire detectives made a connection. Mr Workman shared a surname with a far more likely target: the extradition judge who had refused to send Berezovsky back to Moscow. The gunman was later proved to be a local man who refused to name who had paid him to carry out the hit.

Robert Workman: Same name as British judge who stood up to Moscow. Blasted to death with a shotgun

Then there was the case of Alex Chapman, the former husband of glamorous Russian spy Anna Chapman, who was unmasked in 2010 as part of a network of sleeper agents operating illegally in America. Alex Chapman cashed in on his cameo in the international drama by selling details of the couple's sex life and an album full of intimate photographs to a newspaper. In May 2015, at the age of 36, he was found dead in an empty house in Southampton, apparently from a multiple-drug overdose.

One of the most distressing cases is that of Dr Matthew Puncher, a respected Government nuclear scientist who had helped put Putin in the dock for Litvinenko's murder. He measured the dose of polonium that killed Litvinenko and there was only one place on the planet where the rare nuclear isotope originated in the quantities he detected – the Russian nuclear reactor at Mayak, on the eastern slopes of the Ural mountain range.

Alex Chapman: Revealed sex secrets of his Russian spy wife Anna. Found dead of massive drugs overdose

After a series of spills caused widespread sickness, mutations and cancer, the Mayak facility had been forced to accept foreign help and, in 2016, Dr Puncher had been put in charge of an Anglo-American team of experts. He and his colleagues knew they had been followed and bugged by the FSB, but something else had happened. Inexplicably, Puncher came home with a deep depression. When police found the scientist's body sprawled on the kitchen floor of his Oxfordshire home with wounds in his neck, arms and stomach in May 2016, there had been no sign of a struggle – all the furniture was perfectly in place.

Thames Valley Police concluded he must have managed to stab and slash himself repeatedly with two separate knives before succumbing to his wounds. And they shut down the case.

Meanwhile, American spy agencies provided MI6 with intelligence suggesting that this was yet another Russian hit. It was possible, they said, that Russia could have driven the scientist to suicide but, in their view, it was likely he had been assassinated.

Under mounting pressure, the Government was finally forced to act and last year announced it would review this chain of suspicious deaths.

Yet the task proved all but impossible for the official investigators. In many cases it turned out that little or no evidence had been gathered. No witnesses had been sought, no CCTV had been captured, and no forensic work had been carried out at the scene.

Matthew Puncher: Helped pin Litvinenko's poisoning on Putin. Stabbed to death with two knives

After six months, the charade was brought to a halt, but not before the then Home Secretary Amber Rudd obtained a secrecy order banning the results of the review from being disclosed at the inquest of one victim.

At least the British Government is now prepared to talk tough in public, even if action has been in short supply. After it emerged that the attack on the Skripals had been conducted by two serving members of Russia's military intelligence agency – Ruslan Boshirov and Alexander Petrov – Theresa May vowed to use the 'full range of tools from across our national security apparatus' to hit back.

'Their attempts to hide the truth,' the then Prime Minister thundered on the floor of the House of Commons, 'simply reinforce their culpability.'

She said nothing, though, about the British Government's complicity in this outrageous series of state-sponsored killings on our own doorstep.