MI6 foreign intelligence agency head Alex Younger came out of the shadows to deliver only his second public address since assuming office four years ago - a period of flaring conflicts with Moscow.
Younger's agency has pinpointed the blame for the March poisoning with a nerve agent in England of former double agent Sergei Skripal on two officers with Russia's GRU military intelligence service.
MI6 also accuses the GRU for hacking the OPCW chemical weapons watchdog in The Hague the following month in an apparent bid to figure out the course of its probe into the Skripal case.
Younger called the poisoning a "flagrant hostile act".
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Skripal survived the attack and Moscow has denied involvement in either incident.
"Mr (Sergei) Skripal came to the UK in an American-brokered (spy) exchange, having been pardoned by the president of Russia - and to the extent that we assumed that had meaning," said Younger.
"That is not an assumption we will make again."
Skripal was included in a 2010 spy swap that followed the discovery in the United States of a Russian sleeper cell whose members included former model Anna Chapman.
He was pardoned by former president Dmitry Medvedev in the middle of a four-year span during which Putin served as prime minister and Russia's de facto leader.
Security analysts think Russia may have tried to kill him in retribution for his reportedly continuing cooperation with European intelligence agencies.
'Our own way'
The legendary animosity between Moscow and London spies stretches back to the Cold War era.
Putin himself was a intelligence officer in the former East German city of Dresden in the years leading up to the Soviet Union's collapse.
Yet it is rare for the head of the MI6 to directly confront Kremlin leaders.
Younger said he was focused on making sure Russia paid a price for any attacks on Britain high enough to ward them off in the future.
"Our intention is for the Russian state to conclude that, whatever benefits it thinks it is accruing from this activity, they are not worth the risk," he told students in a talk at his old university of St Andrews in Scotland
"We will do this in our own way, according to our laws, and our values," he said.
"We will be successful nonetheless, and I urge Russia or any other state intent on subverting our way of life not to underestimate our determination and our capabilities, or those of our allies."
Younger did not spell out what changes the MI6 was making in the wake of the Skripal incident.
He only referred obliquely to the development of a "fourth generation of espionage" that worked to counter foes who took advantage of the "blurred lines" of cyber security.
He added that Britain more broadly needs adjusting to a new political reality in which "power, money and politics is going east".
Younger also took pains to dismiss suggestions that Britain's capabilities would weaken once it breaks off from the European Union in March.
"We will always work with our sister agencies to strengthen our indispensable security ties in Europe," said Younger.
"We also work with other partners across the world, to disrupt terrorist activity and counter other serious threats - but always on our terms, and based on our laws and our values as the United Kingdom."
