The Government is under pressure to publish the evidence it submitted for the collapsed China espionage case.
The Conservatives and the Liberal Democrats have both called on the Government to release the evidence provided to the Crown Prosecution Service (CPS) concerning the case of Christopher Cash and Christopher Berry for public scrutiny.
The men were accused of passing sensitive intelligence from inside Parliament to the Chinese Communist Party.
Both denied the charges before the case was dropped last month, as prosecutors said the Government failed to provide enough evidence that China was a threat to national security.
Dan Jarvis, the security minister, said on Monday that Matthew Collins, the deputy national security adviser, had been given "full freedom" to provide evidence to the CPS .
The BBC reported that senior government figures had suggested that the CPS had told them doing so would be "inappropriate".
But the CPS said that now the criminal proceedings had collapsed, it was entirely up to the Government on whether to release the evidence, adding: "The statements were provided to us for the purpose of criminal proceedings, which are now over.
"The material contained in them is not ours, and it is a matter for the Government, independently of the CPS, to consider whether or not to make that material public."
It is the second time the CPS has spoken out about the China spy case. Last week Stephen Parkinson, the director of public prosecutions, said he decided to make a rare intervention because "government briefings have been provided, commenting on the evidential situation".
Labour provoked a backlash from the Civil Service by naming Mr Collins, an unelected civil servant, as the person responsible for the evidence in the case in the Commons on Monday.
Lord Case and Lord Sedwill, who together led the Civil Service for six years, have led a mutiny defending Mr Collins.
Lord Case, who worked with Mr Collins and served under Sir Keir Starmer last year before leaving Downing Street, said: "Matt is an outstanding official with huge integrity and many years [of] experience of dealing with the most sensitive intelligence matters. And he knows more about the serious threat China poses to the UK than most."
Lord Sedwill, who led the Civil Service from 2018 to 2020, said: "Matt shouldn't have been in the firing line, but credit to PM for putting this on the record."
Another source familiar with discussions said civil servants were "furious" and believed Mr Collins had "been totally thrown under the bus" to protect Jonathan Powell, the national security adviser.
Dame Priti Patel, the shadow foreign secretary, who has worked with Mr Collins, said it was "completely disrespectful to hang out to dry a distinguished public servant who is incredibly well respected and has advised successive governments".
Mr Collins was charged by the previous government with giving a witness statement that would satisfy the legal condition that China was an "enemy" of the UK at the time the alleged spying took place.
Chris Philp, the shadow home secretary, said: "Since this evidence would have been cross-examined in court, it cannot be secret.
"The Government should now urgently release Matt Collins's witnesses statements and the correspondence around them in the interests of full transparency.
"Otherwise, there will be legitimate questions about what exactly the Government is hiding."
Calum Miller, the Liberal Democrats' foreign affairs spokesman, said: "The Government must publish the full witness statements it sent to the CPS without delay. The CPS itself has made clear there is no excuse not to do so.
"If ministers have nothing to hide they have nothing to fear. Failure to come clean will just confirm people's suspicions of a cover up and that ministers are more worried about cosying up to China than protecting our national security."
Sir Keir will face further scrutiny over the Government's handling of the China spy case at Prime Minister's Questions on Wednesday afternoon.