A case involving two men accused of spying for China collapsed because evidence could not be obtained from the government referring to China as a national security threat, the UK’s most senior prosecutor has said.

Charges against Christopher Cash, 30, and Christopher Berry, 33 – who both deny the allegations – were dropped by prosecutors last month prompting criticism from ministers and MPs.

In a rare intervention, the Director of Public Prosecutions (DPP) Stephen Parkinson said the Crown Prosecution Service tried to obtain further evidence from the government “over many months” but witness statements did not meet the threshold to prosecute.

Sir Keir Starmer said the government could only draw on the previous government’s assessment which dubbed China an “epoch-defining challenge”.

Mr Parkinson said while there was sufficient evidence to prosecute at the time charges were brought in April 2024, a precedent set by another spying case earlier this year had then raised the threshold needed to convict people under the Official Secrets Act.

China would have needed to have been labelled a “threat to national security” at the time of the alleged offences by Mr Cash and Mr Berry, he said.

The government has always maintained it is “frustrated” the trial collapsed and Mr Parkinson’s intervention comes after weeks of speculation about why the prosecution could not continue.

Sir Keir said that the government’s description of China could not change retrospectively and had to be based on “the position of the last government”.

Mr Berry, a teacher from Witney in Oxfordshire, and Mr Cash, a parliamentary researcher from Whitechapel in London, were arrested in March 2023 as part of an investigation that involved counter-terror police.

They were accused of gathering and providing information prejudicial to the safety and interests of the state between December 2021 and February 2023.

Under the Official Secrets Act, anyone accused of spying can only be prosecuted if the information they passed on was useful to an enemy.

Mr Parkinson said in a letter last month that “the case could no longer proceed to trial since the evidence no longer met the evidential test”, adding that he could not go into the reasons why.

In his latest letter, sent to MPs, he said he was taking the “unusual” step of sharing more details because “government briefings have been provided commenting on the evidential situation”.

The need to describe China as a threat to national security in order to prosecute emerged during a separate spying case earlier this year of six Bulgarian nationals who had been spying for Russia. They were found guilty under the Official Secrets Act.

The act says that a person is guilty of espionage if they act in a manner “prejudicial to the safety or interests of the state” and passed on information that might be “directly or indirectly useful to an enemy”.

Mr Parkinson said this case of the Bulgarian nationals clarified that an enemy under the terms of the Official Secrets Act “includes a country which represents at the time of the offence, a threat to the national security of the UK”.

This meant “further evidence should be obtained” from the government in order to proceed with the prosecution but he added that the evidence was not “forthcoming”.

“Efforts to obtain that evidence were made over many months, but notwithstanding the fact that further witness statements were provided, none of these stated that at the time of the offence China represented a threat to national security,” he said.

Former chairman of the Intelligence and Security Committee and former attorney general Dominic Grieve told BBC Radio 4’s Today programme it seemed “somebody just missed something” as at the time the prosecution commenced there “needed to be focus on whether China was going to be described as an enemy or not”.

He added: “It may matter less how you describe it in official government documents if you are prepared to put the evidence forward at a court case to explain why a country in fact constitutes an enemy status.

“My impression from reading what the DPP – and from what Keir Starmer – has said is that there simply seems to have been a muddle. When this case started, there was a failure to get to grips with what might be required to be proved in court.”

Grieve also spoke about an Intelligence and Security Committee report published in July 2023 which said China has penetrated “every sector” of the UK’s economy.

He added: “It makes very sensible reading and I think it leaves one in absolutely no doubt that China’s activities are on such a scale against the United Kingdom that I would describe them as an enemy.”

The Sunday Times previously reported senior Whitehall officials had met to discuss the trial early last month before the charges were dropped.

Number 10’s press secretary said on Monday “the suggestion that the government withheld evidence, withdrew witnesses or restricted the ability of a witness to draw on a particular bit of evidence are all untrue”.

Since last year’s general election, the Labour government has shown signs of wanting to re-set ties between the UK and China and is conducting a cross-Whitehall “audit” of Britain’s relationship with the country.

In October 2024, David Lammy became only the second foreign secretary in six years to visit China, where he said Beijing and London should “find pragmatic solutions to complex challenges”.

The UK’s national security adviser, Jonathan Powell, one of Sir Keir’s most senior advisers and political allies, also visited the country earlier this year.

The trip, which took place in July, was not announced by the UK government, but became public when the Chinese government released details of a meeting between Mr Powell and Chinese Foreign Minister Wang Yi.

Beijing said Mr Powell had “expressed Britain’s willingness to enhance dialogue and communication with China to build a stable, practical and long-term partnership”.