Former MI5 officer Mr David Shayler vowed to clear his name when he left jail on early release.
Mr Shayler (37), who was jailed for breaching Britain's Official Secrets Act, was released from Ford open prison in Sussex this morning.
Civil rights group Liberty, whose director Mr John Wadham is Shayler's solicitor, issued a short statement in which the former MI5 officer vowed to "fight on".
"My conviction and imprisonment violated both my right to a fair trial and my right of freedom of expression and I intend to fight on," Mr Shayler said in the statement.
"I have asked the Court of Appeal to quash my conviction because of the unfair procedure at the trial. My conviction and sentence were also in breach of freedom of expression - I was imprisoned for telling the truth about the incompetence and illegality in the Security Service.
"The Government will have to justify the fact that it is a crime to report a crime when my case comes before the European Court of Human Rights," the statement said.
The former spy, who served less than seven weeks in jail, will spend another seven weeks electronically tagged under the Home Office's home-detention curfew scheme. He will have to observe a curfew from 7.00 p.m. to 7.00 a.m.
PA