He said that prosecutors had acted in good faith in attempting to secure convictions against Christopher Cash and Christopher Berry – both of whom deny any wrongdoing – but that the Official Secrets Act 1911 posed a “very significant problem”.

He said that he had “no doubt” the case would have gone to trial if legislation had been revised during the period of alleged offending between 2021 and 2023.

Speaking before Westminster’s Joint Committee on the National Security Strategy (JCNSS) yesterday, Lord Hermer said the use of the term “enemy” in the Act created difficulties for prosecutors.

He said: “Standing back in this case, where’s the problem? What is it, because I think we should all be concerned to try and identify what lessons can be learned.

“I’m clear in my mind that certainly one very significant problem here was the Act. The Act wasn’t fit for purpose, it was out of date.”

The Crown Prosecution Service (CPS) dropped the case against Mr Cash, a former parliamentary researcher, and Mr Berry, citing insufficient evidence. The men had been accused of passing sensitive information to Beijing.

The failure centred on evidence from Matt Collins, the deputy national security adviser, who declined to describe China as a “threat” to national security. Prosecutors said that stance prevented the case from proceeding. On Monday, Mr Collins said he had provided evidence of “a range of threats” from China but had not labelled the country a “generic” threat, reflecting the position of the then Conservative government.

Lord Hermer told MPs that the National Security Act 2023 had addressed this problem by replacing the term “enemy” in the old law with a requirement to show that “information was being passed to a foreign power”.

He told the committee: “I don’t understand why it took Parliament so long to pass that. Had that Act been in force at the relevant time for this case, between (2021-23), I have no doubt that the prosecution would have proceeded to trial.”

He also said that Kemi Badenoch’s previous comments as business secretary — that Beijing should not be described as a “foe” — would have undermined the prosecution’s position.

However, Lord Hermer urged against describing the collapse as “farcical” or a “mistake”. “It is easy, particularly when we are all so disappointed about the outcome, to jump to conclusions that may prove to be unfair,” he said.