Shortly after lunchtime, as drizzle fell outside at the start of a week without work, Paddy Colby told his father that he was going to watch television in his bedroom. He trudged upstairs, closed the door and killed himself.

The cheerful handyman and stepfather did not mean to die. Once in his bedroom, he took out a ‘wrap’ of heroin he had bought – a drug which sells on the streets of Hull for £10. Then he put the powder in citric acid, heated it up and injected it into his body.

After years of addiction, Colby, 39, might have noticed the drug was more soluble than usual and the liquid had a strange reddish hue.

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But it hit him so fast that when his stepmother found his body three hours later, the syringe was still in his hand.

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Flashback of the living dead: Ian Birrell's report in August about the effects of the opioids in Ohio. Above, an officer syringes Narcan – a drug that counters opioids – into the slumped woman’s nose

For the ‘wrap’ he bought was not just heroin. It had been mixed with two lethal man-made opioids – fentanyl, a painkiller 100 times more potent than morphine; and carfentanyl, an elephant tranquilliser 10,000 times stronger than street heroin.

These are the synthetic drugs carving such a deadly course across the United States, an epidemic that left 64,000 people dead last year, including rock star Prince. This year’s death toll may double.

Now the drugs have arrived in Britain – and a spate of sudden deaths in Hull, the worst incident in the UK so far, shows their devastating impact. Just a few grains of carfentanyl – 0.00002g – can be fatal.

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Colby was one of at least 15 people – and possibly more than 20 – killed in Hull and the surrounding area in just a few weeks.

Their lives ended after heroin was mixed with the synthetic opioids, which are made in laboratories abroad, bought on the internet and sold at vast profit. In another case, six people were killed on one side of a street in Stockton, Teesside, earlier this year.

These lethal drugs have begun cropping up across the country – first found in Blyth, Northumberland, then suspected in deaths and drug busts from Leeds to London, St Albans to Southampton, Wakefield to Winchester, and Wales to Northern Ireland.

Lethal injection: Heroin mixed with powerful synthetic painkillers is thought to be responsible for a spate of sudden deaths. (Picture posed by model)

This feels eerily familiar to me after reporting two months ago from the front line of the US drug crisis in Ohio – not least since there are signs Britain is seeing the same overuse of prescription opioids.

As the Hull inquests unfold, exposing sad stories of lives wrecked by addiction, events in this proud fishing community should serve as a wake-up call to complacent authorities.

‘These new drugs are the very dangerous tip of an iceberg,’ said Detective Chief Inspector Paul Kirby, of Humberside Police, who admitted he knew little about the problem before it hit his patch. ‘The country needs to be alert.’

The first local warning sign came last September in Goole, when a man released from prison died unusually quickly after injecting drugs. Fatal heroin overdoses usually take several hours, death coming from respiratory failure.

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Early this year there was talk of super-strength heroin on the streets – then came 31 drug-linked deaths in Hull and East Yorkshire by the end of May. Tests confirmed synthetic opioids in 15 bodies, most with a cocktail of carfentanyl and fentanyl.

The link to legal prescriptions Prince died last year aged 57 from a fentanyl overdose The US drug epidemic is rooted in the over-prescription of opioid painkillers, drugs which were heavily promoted by pharmaceutical firms and doled out by doctors like sweets. This led to a big pool of addicted people – many with no history of hard drug use – who shifted to street dealers when prices rose or supplies dried up. Now hundreds die each week from potent synthetic opioids. The best-known victim was rock star Prince, who died last year aged 57 from a fentanyl overdose. The star suffered chronic knee pain from years of high-energy performing. I’m A Celebrity and Britain’s Got Talent presenter Ant McPartlin was briefly hooked on another legally prescribed painkiller. Alarmingly, studies suggest American users often now go straight to street heroin. I’m A Celebrity and Britain’s Got Talent presenter Ant McPartlin was briefly hooked on another legally prescribed painkiller

‘It was shocking to see so many people suddenly dying,’ said one key local source, adding that two corpses had deteriorated too much for testing by the time they were discovered.

‘Probably another four to six of the dead had used the new drugs.’

Last week came the latest in Hull’s series of inquests. Steven Farniss, a 34-year-old with mental health problems, was found dead in a snicket surrounded by drug paraphernalia. His body contained traces of heroin, fentanyl and carfentanyl.

The arrival of these highly toxic substances is especially alarming for Britain since it has the highest proportion of heroin users in Europe and already accounts for almost one third of the entire continent’s overdose deaths.

The UK is also the biggest host of fentanyl sales on the secret ‘darknet’ in Europe, with 1,000 trades made in the last few months alone, according to the Oxford Internet Institute.

Experts say the drug is increasingly appearing for sale online. Two men were jailed last month for importing and selling the drug online in the first such British case.

Profits can be huge – with thousandfold mark-ups of fentanyl bought from China – while it is easy to hide, given the minuscule quantities needed.

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‘You can get enough of some of these drugs in a single envelope to supply millions,’ said Steve Rolles, senior policy analyst at the Transform Drug Policy Foundation. ‘Why smuggle ten kilograms of heroin when you can bring in a matchbox of carfentanyl?’

Such is its potency, US dealers have died from toxic inhalation while cutting supplies. While I was in Ohio, three hospital nurses needed rapid treatment after losing consciousness while treating an overdose patient.

Colby’s tale is tragic but typical. He began using drugs as a young man, part funded by £20,000 compensation from injuring his leg on a fairground ride, and claimed the fatal dose was ‘one last hit’.

He tried to defeat his demons – seeking treatment, taking a heroin substitute and spending four months in rehab at the end of last year before returning to live with his family. The inquest heard there was just a single needle mark on his corpse.

‘In my eyes he has been murdered,’ said his stepmother June Colby. ‘Yes, he has gone out and paid for heroin but he hasn’t paid for this.

‘Those who sold it to him must have known what was in it and they must have known what it could do. What hurts us all is that Paddy had just come from rehab.

‘We were told his body was really healthy and clean, which is why this stuff killed him so quickly.’

Colby’s best friend, Phil Hutchinson, 37, said: ‘He was a great bloke and a fantastic dad and uncle. He would always be doing stuff with the kids. He was nothing like the stereotype of an addict – he’d never burgle anyone’s home.’

But such is the cruel grip of addiction that in the centre of the UK’s 2017 City of Culture, I found Colby’s cousin Danny Colby, 37, who survived a fentanyl overdose but still used heroin. He first took the drug aged 12 when given it by a relative who was dealing.

He told me he bought ‘a bag and a half’ a couple of months ago.

‘I remember taking it with my girlfriend but not falling to the floor. I don’t even remember the buzz. But when I woke five hours later my girlfriend looked like she was dead.’

This affable man admitted the couple were lucky to survive, unlike several friends who died among the carnage earlier this year. ‘I may be an addict but I don’t want to use that again – it’s scary and very bad stuff.’

Others were more phlegmatic. Mark Stephenson, 45, another long-term user, pointed out to me the busy shopping street where he collapsed after taking the adulterated heroin. ‘Next thing I knew I was in an ambulance.’

Yet he said he would take fentanyl again. ‘I don’t care about my life – look at it,’ he said as we sat on blankets below an iconic Hull mosaic on a shuttered shopfront. ‘I don’t want to die but if I do I’m not bothered.’

Not all victims are homeless human wrecks. Britain’s death toll from fentanyl includes an Army captain secretly using cocaine and heroin to handle stress.

And a family in Kent will today scatter the ashes of an 18-year-old skateboarder who died being given a free sample of fentanyl when buying cannabis – a chilling echo of tactics used by US dealers to snare fresh customers.

In August, the National Crime Agency urged families to be vigilant after 60 fentanyl-linked deaths. That figure has now risen to 88 – but these drugs are difficult to detect in blood while forensic tests do not routinely search for them in post-mortems.

In East Yorkshire, a recent audit of just one GP practice in a struggling coastal town found 1,223 people on long-term use of opiates out of 14,000 patients

There are also dozens of different synthetic opioids, making tracking them even tougher. ‘People don’t know what to look for and that’s what is so terrifying,’ said Vicky Harris, a Humberside public health official.

Alarmingly, Britain has also witnessed a sharp rise in prescription opioids. This lay behind the US epidemic – people who became hooked after seeking medical help then turned to street dealers when supplies were stopped or prices rose. Over the past decade, the number of prescriptions issued for opioids has doubled nationally.

In East Yorkshire, a recent audit of just one GP practice in a struggling coastal town found 1,223 people on long-term use of opiates out of 14,000 patients.

To put that in perspective, there are more than 9,000 GP practices in the UK, while the whole of East Yorkshire has about 600 people known to be dependent on drugs.

‘We are creating an opiate-dependent population,’ said Tony Margetts, substance misuse manager for East Riding of Yorkshire Council. ‘It looks to me like a slow-motion catastrophe.’

He said many patients in the audit were middle-aged, moving from industrial areas. ‘Opium feeds on unhappiness and there are lots of older, unhappy people who start taking drugs for pain but enjoy the feeling of calm.’

The arrival of potent synthetic opioids will also intensify debate over how to stop drug use.

Among those I met on Hull’s damp streets was Joe Hood, 44, from Newcastle, who started using drugs in his late 20s after the death of his eight-year-old son from leukaemia.

Hood is another shaken survivor of fentanyl. ‘I remember pushing in the pin [needle] and waking up seven hours later with it still in my arm. I hope I never take it again – but when you go to a dealer you don’t know what you’re getting.’

Hood was in a cemetery when he injected himself. He was fortunate to wake up. Many others will not be so lucky.

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Additional reporting: Ross Slater