The UK has condemned Beijing as the US charged Chinese hackers accused of attempting to steal trade secrets and technologies in a bid to compromise government computers.
More than a dozen allies affected – including the UK – have condemned China over the allegations.
The US Justice Department revealed criminal charges against two hackers allegedly affiliated with Beijing's main intelligence service over a cyber-spying campaign targeting the US and others countries' networks.
The charges accuse a group – known as APT 10 – of spying on some of the world's biggest firms by hacking into technology companies which managed email, storage and other computing tasks.
Countries affected have accused China of backing the large-scale operation, known as Cloudhopper.
In a statement, the Foreign Office said: "This campaign shows that elements of the Chinese government are not upholding the commitments China made directly to the UK in a 2015 bilateral agreement.
"It is also inconsistent with G20 commitments that no country should conduct or support ICT enabled theft of intellectual property, including trade secrets or other confidential business information."
A senior Whitehall source told Sky News it was "certain that UK and allies' trade secrets have been stolen" by APT 10.
The alleged hacking group, which is also known as Stone Panda and Menupass, is a Tianjin based group which has been operational since 2009, according to Sky sources.
Its members are not conventional state employees, but condoned and working in concert with Chinese state security, the source told Sky News.
"They are more like a corporation than a gang," the senior Whitehall source added.
"The tentacles of this campaign are vast. It involves very widespread targeting of globally significant companies, including 'household names' – and this campaign is still ongoing."
Foreign Secretary Jeremy Hunt said the hacking campaign was "one of the most significant and widespread cyber intrusions against the UK and allies uncovered to date".
"These activities must stop," he said in a statement.
"They go against the commitments made to the UK in 2015, and, as part of the G20, not to conduct or support cyber-enabled theft of intellectual property or trade secrets.
"Our message to governments prepared to enable these activities is clear: together with our allies, we will expose your actions and take other necessary steps to ensure the rule of law is upheld."
Theresa May's spokesman later said that the UK enjoyed a strong relationship with China, which allowed it to express disagreement following the hacking claims.
However, he did say that despite requests and time for an investigation, China has "not engaged".
Speaking about the alleged hacking, he said the threat is not thought to be targeting citizens and that he did not yet have information on how many UK businesses may have been targeted.
Meanwhile in the US, FBI director Christopher Wray said "no country poses a broader, more severe, long-term threat" to the American economy than China.
US deputy attorney general Rod Rosenstein said China had "violated" its 2015 agreement to curb cyber espionage for commercial purposes.
More from china
According to a court filing, government agencies and corporations targeted included the US Navy and NASA.
The Chinese nationals charged have been identified as Zhu Hua and Zhang Jianguo.
[contf] [contfnew] 
Sky News
[contfnewc] [contfnewc]
