The statement was made by his lawyer, John Wadham, who spent an hour and half with Mr Shayler at La Sante prison in southern Paris.
The announcement came at the same time as it was revealed that police had searched hotel room of former MI6 agent Richard Tomlinson in New Zealand.
Mr Tomlinson, 35, was served with an injunction earlier this week to prevent "damaging disclosures" about his four-year career with the intelligence service, which took him to Bosnia and Moscow.
The civil rights group Liberty said he was prevented from boarding a plane from New Zealand to Australia and returned to his hotel room as the search was carried out.
Britain wants to extradite Mr Shayler for disclosing information on MI5 operations to the Mail on Sunday newspaper. The Government obtained an injunction last August to stop the paper from publishing further articles.
Mr Wadham, director of Liberty, said outside the prison: "He'll definitely plead not guilty ... He believes he has a case and is prepared to wait in prison."
He said a hearing was unlikely before September or October and that Britain has 40 days to set out the details of its case that Mr Shayler breached the Officials Secrets Act.
Mr Shayler, who left MI5 last year, had access "to highly sensitive information which he has undertaken never to reveal", according to the Home Secretary, Jack Straw.
The Sunday Times reported that Mr Shayler was planning to divulge further details of MI5 operations on the Internet and was working with Mr Tomlinson, a former MI6 agent who served a year in prison after admitting giving secret information to a publisher.
Mr Tomlinson, who has dual British and New Zealand nationality, was jailed for 12 months in December last year after admitting trying to sell his story to an Australian publisher. He was released on licence in April.
French authorities questioned him in Paris over the weekend, at the same time of Mr Shayler's arrest.
But Mr Tomlinson was released and travelled to New Zealand.
A Scotland Yard spokeswoman said: "New Zealand police, acting upon a request from the Metropolitan Police service, searched a premises in Auckland occupied by a 35-year-old man in connection with inquiries in relation to the Official Secrets Act. Two Metropolitan Police service officers were present during the search of the premises."
Mr Wadham who also represents Mr Tomlinson said his client had been trying to start a new life in New Zealand.
Mr Tomlinson said: "Many of these items had already been seized and then returned to me by the French police."
He added that the presence of British officers at the scene was a "waste of taxpayers' money".
A temporary or interlocutory injunction has been granted against him by the New Zealand courts pending their decision on allowing the measure to be applied on a more permanent basis.Reuse content