Legal source for UK Internments


I have a family branch that emigrated from Germany to England in the 1936 - 1939 period. Around 1940, resident aliens in England (and I assume in other parts of the UK) were classified by local tribunals as either exempt from internment or interned. All of the family I'm researching, except one, were classified as exempt from internment. The exception was a 19-yr old man living and working in London since 1936. He had been sent by his parents to attend school, and was stil a student. The unfortunate young man was put aboard the infamous Dunera and transported to Australia, where he was interned for two years. In the middle of WWII, the UK decided they needed more manpower in the army, and gave the interned aliens the option to join the British Army in lieue of imprisonment, and a path to British citizenship. This young man took that opportunity, eventually returned to London and qualified as a Charted Accountant (similar to our CPA).

My question pertains to the legal basis for the roundups and internments. Was there an act of Parliament, or an emergency proclaimation by the PM? I've searched the UK Archives and found nothing pertaining to the legal basis. The digitized tribunal cards for exemptions are available at the UK archives, and the Australian Archives have digitized the complete record for this young man's internment, but I've found nothing about the British legal mandate for the internments in either place..
I am hoping that some members across the pond can provide suggestions of where to look.

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