Diplomatic communications by the European Union (EU) have been successfully intercepted by hackers over many years.
The breach was reportedly discovered by the cyber-security company Area 1.
According to reports by the New York Times, the hackers were able to access thousands of messages where diplomats discussed subjects ranging from the President of the United States of America Donald Trump to global trade.
European officials say that information marked as confidential and secret was not affected by the three-year hack.
Speaking to the New York Times, one expert said that the methods used by the hackers were similar to those previously used by the Chinese military.
“After over a decade of experience countering Chinese cyber-operations… there is no doubt this campaign is connected to the Chinese government,” he said.
The intercepted messages, known as diplomatic cables, reveal one exchange where diplomats describe the meeting between Mr Trump and his Russian counterpart Vladimir Putin in July as “successful (at least for Putin)”.
Another diplomatic cable gives details of a private meeting between Chinese President Xi Jinping and European officials that took place earlier this year.
It quotes Mr Xi as saying China “would not submit to bullying” from Washington “even if a trade war hurt everybody”.
These comments echo a speech he gave on Tuesday in which he said “no-one is in a position to dictate to the Chinese people what should or should not be done”.
A number of other institutions, including the United Nations, were also reportedly affected by the breach and have since been alerted.