Gwendolen Morgan, of law firm Bindmans, representing Mr Miranda, said outside court that the Home Office and police had made "sweeping and vague assertions about national security".
She said Mr Miranda "does not accept the assertions they have made".
He "looks forward to the opportunity to have the Government's actions and assertions fully scrutinised" at the hearing in October, she added.
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Police in New York are still investigating what the substance is and whether it could be harmful.
The Sherlock actor made the comment while he received an award for his humanitarian work.