China's Response

Thanks to Richard for pointing out China's response:

China vehemently denied that its army was involved in international computer espionage on Thursday after newspaper reports that the British government had sustained cyber attacks from the Chinese.

"Saying that the Chinese military has made cyber attacks on the networks of foreign governments is groundless and irresponsible and are a result of ulterior motives," foreign ministry spokeswoman Jiang Yu said.

(emphasis my own)

At this point, there isn't sufficient evidence (in the press) that this isn't true.