Jeremy Fleming, the director of GCHQ, said that the Salisbury nerve agent attack "demonstrates how reckless Russia is prepared to be".
Mr Fleming, who has worked in intelligence for over two decades and was the deputy director-general of MI5 before joining GCHQ, said: "We must not, and have not, forgotten the old foes."
He said the attack on Yulia and Sergei Skripal by a military-grade nerve agent in Salisbury was "a sobering thought" and showed "how comfortable [Russia is] at putting ordinary lives at risk".
He went on to say that Russia is "not playing to the same rules… They're blurring the boundaries between criminal and state activity."
:: UK and allies say Russia behind NotPetya attack
The Salisbury nerve agent attack "demonstrates how reckless Russia is prepared to be," said Mr Fleming, who admitted that the Government has monitored and countered the "growing threat" of Russia "for over 20 years".
Yulia Skripal has turned down an offer of help from the Russian Embassy and told her outspoken cousin not to contact her.
In response, the Russian Embassy in London has speculated that Ms Skripal's statement was fraudulent and created by the Metropolitan Police – a consistent tactic by the Kremlin to avoid addressing questions about its illicit activities.
Speaking at the National Cyber Security Centre's CyberUK event in Manchester, Mr Fleming opened by paying tribute to those affected by the terror attack at Manchester Arena last year.
"I'm sure we can all recall the moment when we realised that this brilliant city, known for its tolerance and inclusivity, was attacked by someone who had neither," Mr Fleming said.
:: GCHQ to open new Manchester base
He applauded the first responders that night and the staff at the security and intelligence agencies who put in long hours in the days and weeks following it.
Mr Fleming also praised the cyber actions against extremists in the Middle East, saying that targeted strikes online had "systematically and permanently degraded" systems linked to Islamic State, which he referred to as Daesh.
"Much of this is too sensitive to talk about in detail, but I can tell you that GCHQ, in partnership with the Ministry of Defence, has conducted a major offensive cyber campaign against Daesh.
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"These operations have made a significant contribution to coalition efforts to suppress Daesh propaganda, hindered their ability to coordinate attacks, and protected coalition forces on the battlefield."
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