The UK government introduced internment in Northern Ireland in 1971 against the advice of its military commander.
New secret documents, released under the 30-year rule, make it clear that the move, regarded as one of the biggest mistakes of the troubles, was made against the counsel of Whitehall advisers.
The decision to detain republican terrorist suspects without trial caused widespread anger in Catholic communities.
Lieutenant General Sir Harry Tuzo, head of the Army in Northern Ireland, had warned it would have a "harmful effect", according to the confidential cabinet papers.