The Russian spy, turned British informer, who was found unconscious together with his 33-year-old daughter on a bench in Salisbury, England, on Sunday, got the first break in his career with Russian Military Intelligence (GRU) when he was stationed to Malta in 1985.

Sergei Skripal and his daughter Yulia remain critically ill and in intensive care after exposure to am”unknown substance”.

MaltaToday has learned that the spy was stationed in Malta after reaching the rank of Colonel in Soviet Intellegence and that he adopted the cover of the Cultural and Sports Attache within the Soviet Embassy in Malta.

He served in Malta into the early 1990s.

In 1995, while on business trips to Malta and Spain, he was approached by MI6, Britain’s Secret Service and he became their ‘mole’, passing them information and the names of dozens of Russian intelligence assets.

Skripal was arrested in 2005 by the FSB (formerly known as the KGB), Russia’s secret services and was reported to have been thoroughly debriefed as to what sensitive information he had passed on to his MI6 handlers.

The biggest question about Skripal’s mysterious poisoning is the timing. If the Russian security services had wanted him to have an “accident” while he was imprisoned, it would have been very easy to organise.

In 2010, he was “swapped” for Russian spies caught in the US, and he immediately settled down in a semi-detached house in Salisbury with his family.

His wife, Liudmila, died of cancer in 2012. Neighbours described him as friendly, sociable and living in full view. He shopped locally, drank in the Railway social club and gambled.

His 43-year-old son died recently while on a trip to St Petersburg in Russia.

The Metropolitan police said that due to the “unusual circumstances” of the attack on Skripal and his daughter, its counter-terrorism unit would now be heading the investigation.

Sources close to British intelligence said toxicology tests would be key in the days ahead. They cautioned that other factors or triggers may have been involved. Samples from the scene are being tested at the military research laboratory at Porton Down. Experts have yet to identify the substance.

Detectives were closely scrutinising CCTV footage. It shows a man and woman – possibly Skripal and his daughter – shortly before 4pm walking together normally past a fitness centre. Soon afterwards both collapsed.

The case has been compared to the 2006 murder of the former Russian intelligence officer Alexander Litvinenko. A public inquiry later ruled this was a state plot, “probably approved” by Vladimir Putin, and carried out on the streets of London by the president’s FSB spy agency.

The British foreign secretary Boris Johnson warned that if suspicions about Kremlin involvement were again confirmed England might pull out of this summer’s World Cup, to be hosted by Russia.

“I think it will be very difficult to imagine that UK representation at that event could go ahead in the normal way,” he told MPs.

He added: “We don’t know exactly what has taken place in Salisbury, but if it’s as bad as it looks, it is another crime in the litany of crimes that we can lay at Russia’s door. It is clear that Russia, I’m afraid, is now in many respects a malign and disruptive force.”

Russia’s foreign ministry dismissed his comments as “wild” and accused the UK of hysteria and unwarranted “Russophobia”. The embassy in London said “there was no truth” in the suggestion its special services were connected.

The GRU (Main Intelligence Directorate) is the foreign military intelligence agency of the General Staff of the Armed Forces of the Russian Federation (formerly the Soviet Army General Staff of the Soviet Union).

Since 2010, the agency′s official full name is the Main Directorate of the General Staff of the Russian Armed Forces .

The GRU is Russia's largest foreign intelligence agency. In 1997 it deployed six times as many agents in foreign countries as the SVR, the successor of the KGB's foreign operations directorate. It also directly commanded 25,000 Spetsnaz special forces troops in 1997.