In his first public speech since taking over as head of Britain's eavesdropping intelligence agency last year, Jeremy Fleming said the poisoning of Sergei Skripal and his daughter in Salisbury, southern England, was "particularly stark and shocking".
He accused Russia of "not playing to the same rules" and blurring the boundaries between criminal and state activity.
Britain has blamed Russia for the attack on the Skripals while Moscow has denied any involvement. It has led to one of the biggest diplomatic crises between Russia and Western nations since the Cold War.
"You've heard it said, and I'll repeat, the attack on Sergei and Yulia Skripal in Salisbury, was the first time a nerve agent had been deployed in Europe since the Second World War," Fleming told a cyber conference in Manchester, northern England.
"That's sobering. It demonstrates how reckless Russia is prepared to be, how little the Kremlin cares for the international rules-based order, how comfortable they are at putting ordinary lives at risk."
Fleming, a former agent in Britain's MI5 domestic spy service, said the robust response from Britain and its Western allies, which led to both side expelling scores of diplomats, showed the Kremlin that "illegal acts" had consequences.
Russia has accused Britain of trying to drum up anti-Russian sentiment, suggested the British might have carried out the attack themselves.
Fleming said Britain had for decades collected intelligence on Russian capabilities and for more than 20 years monitored their cyber threat.
"It looks like our expertise on Russia will be in increasing demand," he added. "We will continue to expose Russia's unacceptable cyber behaviour."
In his speech, Fleming also revealed that GCHQ had carried out a major cyber offensive campaign against Islamic State (IS)militants, the first time, he said, Britain had systematically and persistently degraded an enemy's efforts as part of wider action.
"These operations have made a significant contribution to the coalition's efforts to suppress their propaganda, hindered their ability to coordinate attacks and protected coalition forces on the battlefield," he said.
"In 2017 there were times when Daesh (IS) found it almost impossible to spread their hate online, to use their normal channels to spread their rhetoric, or trust their publications."
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