Up to a dozen undercover Russian agents are operating in the UK under Vladimir Putin’s command, security officials claim.
The highly-trained Russian operatives are said to be operating in two-person sleeper cells, as a crackdown on spy activity in the UK gets underway.
It follows a bombshell report into potential Kremlin interference in the Brexit referendum which sparked calls from ministers to strengthen security laws.
The report by MPs accused the government underestimating the threat of Russian interference.
The Intelligence and Security Committee report also claimed the Tory government made no effort to investigate claims of Russian interference in the EU referendum.
In response the government has vowed to root out the undeclared agents by making their presence in the UK illegal as it is not currently a crime to live in the UK as an undercover officer.
This will mean that amendments take place to the Official Secrets Act with spies risking being prosecuted and jailed if caught out.
Such so-called “illegals” have been trained in spy schools outside Moscow and could be being used to monitor activity in the UK.
The sources, who have not been named, believed the agent’s spy skills could be utilised for pre-mission surveillance, potentially including the novichok attack on Sergei Skripal.
Referring to the Russian spy service, a former military intelligence officer told The Mirror : “It is remotely possible the GRU were helped in the Salisbury attack by an illegal.”
It was suggested the agents could provide data on coastal terrain or local troop movements.
Crucially they would also know what to look for in critical infrastructure such as dams, water systems and power networks or installations to be targeted for sabotage.
Russia’s most celebrated secret agent of recent years was Anna Chapman, who remained undercover for many years in Britain and the US before being rumbled by the FBI in 2010.
The Government is also invoking the Magnitsky Act, named after murdered Moscow lawyer Sergei Magnitsky, which can allow the freezing of Russians’ assets.
Prime Minister Boris Johnson said: “This Government is bringing forward legislation, not only a new Espionage Act, not only new laws to protect against theft of our intellectual property but also a Magnitsky Act directly to counter individuals in Russia or elsewhere who transgress human rights.”