The rule that a country cannot deport a suspected non-national terrorist lest that person be subjected to torture or to inhuman or degrading treatment or punishment by the receiving country is not expressed in plain terms in the European Convention on Human Rights nor in any of the amending Protocols to that treaty.
It was created by the Court of Human Rights in the case of Chahal v UK decided in 1997, and the judges divided by 12 votes to 7 on the point. There are a number of reasons for treating the case as suspect.
The first is that it was decided before the change in perspective on international terrorism brought about by the events of 11th September 2001. Judges like Felix Frankfurter of the United States have noted that in times of heightened security risks to the community as a whole “the claims of government, based upon any reasonable construction, must trump individual rights, unless those rights were essential to the broader historical framerwork of republican constitutionalism” (1).
The second is that the Court of Human Rights did not consider its responsibilities under Article 17 of the European Convention, which in effect denies the protection of the Convention to those who would destroy the rights and freedoms protected by it (2). The right to life is such a right.
The third is that the Chahal case was decided the wrong way around. He had lived in the UK for 19 years, and had founded a family in the UK. Proposing to deport him (and his family) was wholly disproportionate, and could and should have been stopped on that ground alone. It is shameful that deportation was even considered.
(1) Article on Frankfurter J by Peter Charles Hoffer
(2) See, for example, Case 250/57