Richard Horne, who became head of the National Cyber Security Centre (NCSC) in October, used his first major speech to warn of "the aggression and recklessness of cyber activity we see coming from Russia".
He also cautioned that China "remains a highly sophisticated cyber actor, with increasing ambition to project its influence beyond its borders" as his agency unveiled its latest annual review.
"Hostile activity in UK cyberspace has increased in frequency, sophistication and intensity," Horne said, with online actors increasingly targeting technology used by people daily "to cause maximum disruption and destruction".
"We believe the severity of the risk facing the UK is being widely underestimated," he added.
"There is no room for complacency about the severity of state-led threats or the volume of the threat posed by cybercriminals.
"The defence and resilience of critical infrastructure, supply chains, the public sector and our wider economy must improve."
The NCSC, which sits within the top-secret electronic eavesdropping agency GCHQ, has repeatedly warned of the growing risks from cyberthreats.
The UK has been hit by an array of disruptive cyberattacks in recent years, with targets including London's transport system, some of the UK's biggest hospitals and the British Library.
Hostile states are suspected of a hand in some of them.
"What has struck me more forcefully than anything else since taking the helm at the NCSC is the clearly widening gap between the exposure and threat we face, and the defences that are in place," Horne said.
"We all need to increase the pace we are working at to keep ahead of our adversaries."
Horne singled out Moscow and Beijing, noting that cyberattacks were "increasingly important to Russian actors, along with sabotage threats to physical security".
His comments follow a warning last week by Pat McFadden, a senior UK government minister whose portfolio includes national security, that Russia was "exceptionally aggressive and reckless in the cyber realm".
A year ago, the UK and United States accused Russian security services of engaging in a sustained cyber espionage campaign against top politicians, journalists and NGOs.
But the latest accusations come with tensions between the West and Russia particularly heightened, with President Vladimir Putin sparking unease last month when he said the war in Ukraine had the characteristics of a "global" conflict.
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