Miles Cooper, 27, was given an indeterminate sentence for sending seven explosive devices made from party poppers and glass or nails through the post.

He targeted offices in London, Berkshire, Oxfordshire, Kent, Birmingham and Swansea in January and February this year as part of a campaign against what he claimed was a "surveillance-obsessed society".

He was found guilty by a jury at Oxford Crown Court yesterday of 11 charges relating to explosive devices.

When Cooper returned for sentence, Judge Julian Hall ruled he was a danger to the public and told him he would serve at least five years, minus 116 days already spent in custody, before he could be considered for parole.

He told him: "First and foremost you are a terrorist, let there be no mistake. Anyone who tries through violence or threat of violence to change the political will is a terrorist and that is precisely what you did."

The jury had heard victims, including a pregnant woman, were showered in glass fragments or nails when they opened the padded A5-size envelopes.

Cooper did not deny sending the letters to three forensic science laboratories, Capita who run London's Congestion Charge, an accountancy firm dealing with speed cameras, the DVLA and a residential address, but did deny intending to cause any injury.

Judge Hall said Cooper's 'cowardly' actions had caused lasting psychological damage to his victims and massive disruption to the organisations he targeted.

Cooper claimed in court his campaign was in protest at Britain's "authoritarian" Government and he resorted to violent means as his peaceful protests against measures imposed by Tony Blair's administration had failed.