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Police are probing links ­between “spy in the bag” victim Gareth Williams and the poisoning of father and daughter Sergei and Yulia Skripal.

Williams, 31, a codebreaker at Britain’s GCHQ eavesdropping centre who was on secondment to MI6, was found dead inside a locked red holdall in his bath eight years ago.

At the time his bizarre death in his London flat was dismissed as a sex game that had gone wrong.

But this week two counter-terrorism detectives from Salisbury, Wilts – where the Skripals were poisoned by a Russian nerve agent – will question KGB defector Boris Karpichkov about Williams’ death.

Karpichkov, 59, claims that Williams was murdered with an untraceable poison because he had discovered the identity of a Russian mole at GCHQ.

(Image: Sunday Mirror)

The former spy told the Sunday People: “Russian security services are the connection between the two cases.”

Karpichkov, a former major in the KGB and its FSB successor, lived near Williams in London’s Pimlico and has made an extensive study of the case.

He claims he received information from a source in Russia’s foreign intelligence agency, SVR, that Williams was murdered because he knew the identity of a Russian mole codenamed Orion in GCHQ at Cheltenham, Glos.

The Russians hoped to recruit Williams as a double agent in an operation codenamed Sweetie by threatening to reveal the spy’s fondness for cross-dressing.

He had £20,000 of women’s clothes stashed in his flat.

(Image: Phil Harris)

Karpichkov says: “Williams had no intention of letting the Russians blackmail him.

“But he was foolish enough to say that he knew the person who tipped them off.”

He added: “The SVR had no alternative but to kill him to protect their agent inside GCHQ.”

Williams and Orion had become friends while at the Government listening post, having joined at around the same time.

But Orion was too important to the Russians to risk exposure.

His spymaster in Britain, an undercover agent posing as a rich East European businessman codenamed Lukas, attempted to blackmail Williams, according to Karpichkov.

When that failed, Lukas tried to make amends by turning up at his flat with a bottle of wine. Williams accepted a glass but it was spiked with a mixture of amyl nitrate and the Viagra drug Sildenafil and he passed out.

A special ops squad known as The Cleaners arrived to finish the job with a plant-based poison made from belladonna, aconite and black henbane mixed with other chemicals.

The killer cocktail was designed to leave no trace at postmortem.

It was shoved into one of Williams’ ears with a ­needle-less syringe.

Kharpichkov had been monitoring and noting the numbers of Russian diplomatic cars which began sweeping through Pimlico from July 2010, fearing he was their target.

Russian spymasters put him under a death sentence when he escaped to Britain 20 years ago carrying two suitcases full of secrets.

(Image: PA)

He thought the drive-bys were the start of a “wet job”, Russian spy slang for an assassination by the FSB’s so-called Liquid Affairs Department.

Karpichkov spotted the last Russian car in the area on 15th August, the day before police think Williams died in his top-floor flat in Alderney Street.

It was only after Williams’s death Karpichkov realised the cars were there for the MI6 man.

Karpichkov adds: “I had never seen those cars before and I never saw them again.”

Karpichkov says an SVR clean-up team later returned to Williams’s flat through a skylight to remove remaining incriminating evidence.

No fingerprints, palm-prints or footprints were found. Nor were any traces of Williams’s DNA discovered on the rim of the bath, the bag zip or the padlock. A key to the padlock was inside the bag, underneath his body.

(Image: PA)

Although an inquest said the death was “likely to have been criminally meditated”, a subsequent police investigation decided it was an accident.

They thought Williams had suffocated when a sex game went wrong.

But Karpichkov says: “Williams was put in the bag so the body could be taken away later. But something must have gone wrong. SVR or FSB killers always try to dispose of bodies so their murders are harder to investigate.”

That, says the spy, is what happened to jet-setting billionaire Leonid Rozhetskin, 41, co-founder of British finance newspaper City AM and pal of British actress Dyan O’Connor.

Rozhetskin, an outspoken critic of Russian president Vladimir Putin, disappeared on March 16, 2008, from his six-bedroom mansion in the Latvian coastal resort of Yurmala.

Four years later his decomposed body was found in woods 90 miles away by a mushroom picker. Karpichkov says an FSB hit squad beat and poisoned Rozhetskin before bundling him into his own 4x4, later found bloodstained and abandoned.

(Image: Patrick McMullan)

Latvian Interior Minister Markes Seglins said all the evidence pointed to murder.

Karpichkov says: “Russian security services claim they stopped killing in the 1950s. But I know the KGB, and then the FSB and SVR carried on killing and do so today and their trademark is poison.” A spy who refused to take part in the killing of defector Alexander Litvinenko, 43, with Polonium-210 became a victim of it himself.

Officially Igor Ponomarev, 41, was Russia’s permanent representative to the International Maritime Organisation in London and died of a heart attack on October 30, 2006, after a night at the opera.

But Karpichkov says he was an undercover SVR agent tasked with organising the logistics for Litvinenko’s murder.

When he refused, a Zaslon death squad from London’s Russian Embassy were despatched to slip him Polonium-210 and a dose of the insomnia remedy Dephenhydramine.

Because of his diplomatic status Ponomarev’s body was spirited out of Britain before any tests could be done.

(Image: PA)

Karpichkov, who was warned in February he was on the same FSB death list as double-agent Sergei Skripal, 66, and his 33-year-old daughter, says: “Every Russian embassy has a Zaslon unit of professional killers for just this kind of thing.

“The SVR wanted Ponomarev dead because he was putting the Litvinenko liquidation operation in jeopardy.”

In November 2010 Putin critic Victor Kalishnikov – a distant relative of the inventor of the AK-47 assault rifle – and his wife were treated at Berlin’s Charity Hospital for suspected mercury poisoning in what they claimed was an attempt on their lives by the FSB.

The Russians have been poisoning their enemies since the October Revolution in 1917.

A 1964 CIA report declassfied in 1993 says: “Even in cases where the Soviet hand is obvious, investigation often produces only fragmentary information due to KGB ability to camouflage its trail.”

But they bungled the attempt to kill the Skripals – and the trail could now be leading back to Gareth Williams.