The Conservatives and the Liberal Democrats are demanding the Labour government publish the evidence it submitted in the now-collapsed case against two people accused of spying for China.
It follows another public intervention from the Crown Prosecution Service (CPS), which has made clear it would not stand in the way if ministers chose to put their evidence in the public domain.
On Tuesday, senior government figures had suggested that the CPS had told them to do so would be "inappropriate".
Prosecutors unexpectedly dropped the case against Christopher Cash and Christopher Berry last month, prompting criticism from ministers and MPs. Both men deny the allegations.
The director of public prosecutions said the case collapsed because evidence could not obtained from the government referring to China as a national security threat.
