Britain's MI5 spy chief said on Thursday he was frustrated by the collapse of a China spying case which has led to intense scrutiny of whether the government was to blame, saying Chinese operatives posed a daily national security threat.

Britain's Crown Prosecution Service unexpectedly dropped charges last month against two British men, former parliamentary researcher Christopher Cash and academic Christopher Berry, who were accused of spying for Beijing between 2021 and 2023.

They had denied passing politically sensitive information to the Chinese state, while Beijing said the case was entirely fabricated.