Russian President Vladimir Putin has reportedly ordered up to 50 sleeper agents in Britain to prepare to strike against the United Kingdom.
MI5 believes Putin has ordered all sleeper agents deep undercover to be ready to spring into action at a moment's notice. They are also worried about attempts to steal the United Kingdom's military secrets and target Russian dissidents and Ukrainian activists living there.
A senior intelligence advisor claims Putin has spies in all areas of society, from top schools to staff in the House of Commons.
"We have to assume Russia is now active at all levels of British society. They scoop up all forms of intelligence and pass it back to the Kremlin through handlers," they told The Mirror.
"This could be anything from what sort of weaponry is being sent to Ukraine - and how much of it - to the sexual antics of the country's political and military leaders."
A former military intelligence officer, Lieutenant Colonel Philip Ingram, said it's impossible to accurately say how many Russian agents are in the United Kingdom.
"Of course there are different types - declared intelligence officers known as part of Russian diplomatic missions, and those operating undercover trying to recruit agents and then sleeper agents in all aspects of society," he said.
"Given our support for Ukraine, Russian intelligence will be focused heavily on operations inside the UK and could also include recruiting agents inside political establishments, defence and industry."
It comes after a former spy claimed there are at least four Russian agents currently working in the UK's Parliament.
Former Russian spy Boris Karpichkov, 62, claimed there are at least four Russian agents currently working in Westminster.
"I know of four sleepers of Russian security services," he told The Sun.
"There are several Parliament-sponsored organisations operating in Westminster directly linked to Russian security service operations."
Last week, a suspected Russian spy in his 40s was arrested at Gatwick Airport as he attempted to leave the UK.
A source told The Sun that he is believed to be in the UK "spying on behalf of the Putin regime".
"He was kept under observation and arrested as he arrived at Gatwick to try and fly out of the country."
