GCHQ now "devotes more resource to China than any other single mission," according to the intelligence agency's director, Anne Keast-Butler.
China poses an "epoch-defining challenge" to the UK, she said at CyberUK, the National Cyber Security Centre's conference in Birmingham.
Last week, Sky News discovered China had hacked the Ministry of Defence's payroll system and in March, Chinese hackers were accused of stealing data about UK voters from the Electoral Commission.
After news about last week's attack, China's foreign ministry said it "firmly opposes and fights all forms of cyber attacks" and "rejects the use of this issue politically to smear other countries".
However, the UK's cyber security leaders are clear.
"China is increasingly working with others to try and reshape the world," said Ms Keast-Butler.
"Through their coercive and destabilising actions, the People's Republic of China (PRC) poses a significant risk to international norms and values," she said.
Responding to China is her "top priority", Ms Keast-Butler added.
She said: "In cyberspace, we believe that the PRC's irresponsible actions weaken the security of the internet for all.
"China has built an advanced set of cyber capabilities, and is taking advantage of a growing commercial ecosystem of hacking outfits and data brokers at its disposal.
"China poses a genuine and increasing cyber risk to the UK."
The threat from China has changed over the last decade, according to Japan's deputy national security adviser Keiichi Ichikawa, who was also speaking at the event.
"Traditionally, China conducted cyber attacks to steal technology, but now it is about cyber attacks against critical infrastructure," he said.
In March, Chinese state-sponsored hacking group Volt Typhoon was caught hacking into US infrastructure projects. The White House's national cyber director Harry Coker said the attack marked a shift in tactics.
Mr Coker said: "This was the first time we saw an entity position itself into our critical infrastructure with the clear intent to disrupt our ability to mobilise against [them].
"[China's] hackers are working on circumventing our defences and targeting our interests at an unprecedented scale.
"They are doing this for one reason, to hold critical infrastructure at risk in a time of global competition.
"In a crisis or conflict scenario, China will wreak havoc on civilian critical infrastructure to deter US mobility."
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Tap hereCyber threats around the UK are increasing dramatically.
An average UK organisation experienced 788 weekly attacks in 2023, according to data from cyber security firm Check Point.
As of early May, that number leapt to 1,078 weekly attacks for an average organisation. That's a 36% jump in just one year.
As elections loom around the world, cyber security experts are ramping up their defences to try to protect nations from hacks.
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