The United Kingdom is devoting more resources to countering state threats than during the Cold War, BBC reports.
Sir Ken McCallum, head of Britain's domestic intelligence service (MI5), made the remarks.
He described Russia as "aggressor number one".
According to McCallum, Russia is carefully waging hybrid warfare against the West while seeking to avoid triggering a collective NATO response.
"The Kremlin would be foolish to cross that line," the security official noted.
He added that the identification and expulsion of Russian agents from Britain had left the Kremlin dependent on paying proxies to carry out sabotage attacks.
"With its traditional covert footprint eviscerated by almost 1,000 expulsions since the invasion, the Kremlin's global toolkit is weak. So we see them scrambling to pay criminals to do their bidding," Ken McCallum stated.
He also noted that Iran's hybrid attacks in Britain have increased.
According to him, there have been 50 arrests over the past year in connection with Iran-linked activities.