The trial of former MI5 spy David Shayler has begun with a vow to put the British state on trial and call cabinet ministers as witnesses.

Shayler, 35, is facing three charges under the Official Secrets Act at the High Court, following alleged leaks to a Sunday newspaper about the work of the Security Service.

Shayler, who was a journalist before joining the service, has claimed that the government was involved in a plot to assassinate Colonel Gaddaffi. After the articles were made public, Shayler went on the run before being arrested in Paris and then returning voluntarily to Britain to stand trial.

Shayler told journalists on Thursday: "We are going to put the British state on trial. I have nothing to fear - but they have everything to fear."

He claimed that because of the political nature of his case it would be delayed until after June. "It's obvious that this trial will be put back as a result of this preliminary hearing, possibly to appeal at the House of Lords. If it takes place it will take place after the general election," Shayler claimed.

"We plan to subpoena Jack Straw and Robin Cook and it is quite obvious they don't want to answer questions in open court right before an election," he added.

Civil rights group Liberty said the case was a vital attempt to reform the laws governing the security services.

Liberty director John Wadham said: "It's an important and the start of an attempt to ensure that the Official Secrets Act is balanced by the principle of freedom of expression. The Official Secrets Act needs reform by parliament. We must have a public interest defence and real accountability of the security and defence services."