Katharine Gun, 29, was told she will not face trial after the Crown Prosecution Service offered no evidence against her.
And the CPS, which refused to go into the reasons for its move, was under mounting pressure to reveal why it had suddenly halted the action against Ms Gun.
Her lawyer, Ben Emmerson QC, said: "Katharine Gun is entitled to know - and, perhaps more importantly, the public is entitled to know."
Ms Gun was arrested last March but it took eight months until charges were laid against her in November. And it took another three months before the CPS told her lawyers yesterday that they would drop the case.
She was charged under the Official Secrets Act of 1989, accused of disclosing security and intelligence information and sacked from her job as a translator at Government Communications Headquarters.
The allegations against her centred on the disclosure of a request allegedly made by a US National Security Agency official.
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