18. Membership of terrorist groups
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(1) Any person who—
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(a) is a member of;
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(b) professes to be a member of,
a terrorist group commits an offence and is, on conviction on indictment, liable to imprisonment for a term of 15 years.
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(2) It shall be a defence for a person charged with an offence under this section to prove that the entity in respect of which the charge is brought was not a terrorist group at or on the date that he or she—
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(a) became a member of; or
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(b) professed to be a member of,
that entity, or that he or she has not taken part in the activities of that entity, after it became a terrorist group.