Conservative leader Kemi Badenoch has written to the prime minister asking him to address “unanswered” questions about the collapsed case against two men accused of spying for China.

Charges against Christopher Cash and Christopher Berry – who deny the allegations – were dropped in September, prompting criticism from MPs.

The director of public prosecutions (DPP) said the case collapsed because evidence could not be obtained from the government referring to China as a national security threat. On Sunday, Education Secretary Bridget Phillipson said ministers were “disappointed” it had not proceeded.

In her letter, Badenoch said the government’s account of the situation had “changed repeatedly”.

Sir Keir Starmer previously said ministers could only draw on the last government’s assessment of China – which dubbed it an “epoch-defining challenge” – and his government has maintained it is “frustrated” the trial collapsed.

Badenoch outlined “several key questions which remain unanswered” in her letter on Sunday, and asked that Starmer or a senior minister appear before MPs “to clear things up once and for all”.

She wrote: “This is a matter of the utmost importance, involving alleged spying on Members of Parliament. It seems that you and your ministers have been too weak to stand up to Beijing on a crucial matter of national security.”

The letter queried remarks made by Phillipson to the BBC earlier in the day, in which she said Starmer’s national security advisor Jonathan Powell had no role in the “substance or the evidence” of the case.

Phillipson also said ministers were “deeply disappointed that the case hasn’t proceeded”, and insisted the Crown Prosecution Service (CPS) was “best placed to explain why it was not able to bring forward a prosecution”.

The Conservatives had suggested Powell, who has sought closer relations with Beijing, failed to give the CPS the evidence it said it needed to secure convictions.

Badenoch questioned Phillipson’s comments: “What does this mean? If he was “not involved” in the decision over months not to give the CPS what they needed, then who was?”

The opposition leader also claimed the government – which had denied ministers were involved in the trial’s collapse before the DPP claimed the necessary material had not been obtained – had sought to “appease China”.

She disputed Starmer’s comments that ministers could only draw on the previous Conservative government’s position, writing: “As various leading lawyers have pointed out, this is not how the law works.”

Starmer had told reporters earlier this week: “You have to prosecute people on the basis of the circumstances at the time of the alleged offence”.

“So all the focus needs to be on the policy of the Tory government in place then.”

Badenoch asked that Starmer clarify whether any ministers knew about the government’s interactions with the CPS in which it “refused” to provide the material being sought.

She also asked if the matter had ever been raised with Starmer, including by Powell, and if an earlier denial of the government’s involvement had been “misleading”.

The Conservatives have submitted an urgent question in Parliament, asking ministers to address MPs on Monday to explain why the trial collapsed.

Shadow home secretary Chris Philp told the BBC ministers “must urgently explain why it chose not to disclose the reams of information it has demonstrating China was a threat to national security in the 2021-2023 period”.

He said: “It looks as if Jonathan Powell was behind this decision – and he should resign if he is.”

Conservative MP Tom Tugendhat, who hired one of the accused men as a researcher when he set up the China Research Group, said it was “absolutely abhorrent” that the case collapsed.

Tugendhat told BBC Radio 4’s Today programme that the government appeared to be “willing to cover up for the actions of a hostile state”.

He added that any accusations around Powell are “simply a diversion from the reality that it is the prime minister who orders, or does not order, special advisers to act”.

Meanwhile, several former Conservative ministers and advisers have told the BBC there was no official designation of whether a country amounts to a threat.

They claim there is a document with “hundreds” of examples of Chinese activity posing a threat to the UK at the time of the alleged offences, which could have been given as evidence.

Sources cited the hack on the Ministry of Defence, which ministers suspected China was behind, as one of many incidents.

“I don’t think there is a sane jury in the world that would look at that evidence and conclude China was not a threat,” a source in the last government said.

Former Conservative ministers also point to public statements, including from the former head of MI5 Ken McCallum, who in 2023 said there had been a “sustained campaign” of Chinese espionage on a “pretty epic scale”.

The Liberal Democrats said the government’s approach to China was “putting our national security at risk”.

The party urged the government to block the planning application for a new Chinese embassy in London.

“Giving the green light to the super embassy being built in the heart of the City of London and above critical data connections would enable Chinese espionage on an industrial scale,” Liberal Democrat foreign affairs spokesman Calum Miller said.

Mr Cash, a former parliamentary researcher, and Mr Berry, were charged under the Official Secrets Act in April 2024, when the Conservatives were in power.

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.

Last month, the DPP said “the case could no longer proceed to trial since the evidence no longer met the evidential test”.

Additional reporting by Maia Davies