European human rights judges would rule Britain’s plan to waive death penalty assurances for two suspected members of the Isil ‘Beatles’ terror cell illegal, experts say, and could order the UK to seek US guarantees and even pay the men damages.

The decision by Sajid Javid, the Home Secretary, has already been challenged with a judicial review in the High Court.

Even if British justices decide the failure to seek guarantees for Alexanda Kotey and El Shafee Elsheikh is legal, a case could be brought to the European Court of Human Rights (ECHR) in Strasbourg, France.

“The Home Secretary’s decision in this case is in the clearest possible breach of the European Convention on Human Rights,” Ben Emmerson QC, the former UN Special Rapporteur on Counter-Terrorism and Human Rights, who currently sits as a judge for the UN International Criminal Tribunals, told The Telegraph.

The Convention has a protocol that abolishes the death penalty in all circumstances and an article guaranteeing the right to life.

Article 3 of the Convention, which forbids “inhuman and degrading treatment” has been used in the past to fight extraditions from Britain to the US because a prisoner would face the death penalty. The Convention is given force in British law in the Human Rights Act 1998.