Case C-118/20 Wiener Landesregierung (Revocation of an assurance of naturalisation):
The revocation of an assurance of naturalisation must have due regard to the principle of proportionality where that revocation prevents the person concerned from recovering the citizenship of the Union.
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However, it is in principle for the Member State the nationality of which the person concerned applied to relinquish in order to be able to obtain the nationality of another Member State to satisfy itself that its decision in response to that application only enters into force once the new nationality has actually been acquired
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Source – CJEU