Steele worked for MI6 at the British Embassy in Moscow in the 1990s, and from 2006 to 2009 ran MI6’s Russia desk in London, The New Yorker reported this week.

In 2016 Orbis was subcontracted to investigate Donald Trump’s links to Russia.

Steele compiled a dossier containing astonishing allegations against the then presidential candidate, including a claim the Russians had cultivated Mr Trump and traded favours with him for at least five years, and handed him intelligence from the Kremlin on Hillary Clinton and other political rivals.

The Telegraph reported that if the Kremlin believed Skripal helped compile the Trump dossier, it could provide a motive for the assassination attempt in Salisbury.

A former Russian construction magnate told Channel 4 News Skripal was “working in cyber-security and every month going to the [Russian] embassy to meet military intelligence officers”.

The Telegraph said a “hit squad” was dispatched by the Kremlin to assassinate Skripal, to send a message that traitors were not tolerated.

UK intelligence services suspect the attack was delayed by the snowstorm that enveloped Britain last week, which kept Skripal and his daughters indoors and made the assassination squad’s escape from the country more problematic.

“By Sunday afternoon, with Miss Skripal on the brink of returning to Moscow, the assassination squad had little room for manoeuvre and were forced into action in broad daylight in the middle of a busy shopping precinct,” the Telegraph reported.

The Trump dossier – a series of memoranda sent by Steele to his contractor in the US - alleged the Russian secret services possessed compromising material that could be used to blackmail Trump, such as a video of him engaging in “perverted sexual acts” in a Russian hotel.

The sources were described in a report in The New Yorker as hotel staff, a close Trump associate, and a former top-level Russian intelligence officer still active inside the Kremlin.

Steele redacted his sources from the dossier, fearing exposure would be a death sentence.

In 2016 he briefed some US reporters about the contents of the dossier. In early 2017 Buzzfeed published the dossier in full.

Last year a lawyer for Glenn Simpson, head of the company that commissioned Steele’s investigation of Trump, told the US Senate Judiciary Committee that “somebody’s already been killed as a result of the publication of this dossier”.

Media speculated this was Oleg Erovinkin, a former Russian intelligence officer and chief of staff to the president of oil giant Rosneft. Erovinkin was found dead in his car in December 2016, and it was later claimed he died of a heart attack.

Around the same time that Erovinkin died, The New Yorker said, Russian authorities charged a cybersecurity expert and two FSB officers with treason.

British police said Skripal and his daughter were deliberately targeted by an unknown assailant armed with a deadly toxin.

"Having established that they were exposed to a nerve agent we are now treating this as a major incident involving an attempted murder by the administration of a nerve agent," the head of Counter Terrorism Policing Mark Rowley said.

A policeman was also hospitalised and was in a serious condition after exposure to the agent. On Thursday he was said to be “talking and engaging” with doctors.

British media have speculated the toxin may have been sarin or VX.

The BBC reported, however, that it was neither of these chemicals, but something “more rare”.

Scientists at the UK's secret weapons research facility at Porton Down, near Salisbury, are looking for chemical clues that may identify its origin.