Citizenship deprivation of foreign terrorist fighters by the United Kingdom is increasing. This is of debatable legality under international law on five separate grounds. First, the UK is arguably wrong in claiming that an extraterritorial deprivation is outside the jurisdiction of the ECHR. Second, UK law may be unlawfully arbitrary and discriminatory. Third, UK law arguably contravenes the Convention on the Reduction of Statelessness 1961. Fourth, the UK may be violating its customary international legal obligation to readmit nationals. Fifth, UK practice may breach its conventional extradite or prosecute obligations. Overall, there are arguments of considerable strength that can be made in opposition to UK law and practice in the area.