Boris Johnson privately warned there could be “reprisal” killings of British citizens if two Isil fighters allegedly members of the infamous ‘Beatles’ terror cell were executed in America. 

Mr Johnson, foreign secretary at the time, expressed the concerns in a June 2018 letter discussing whether the UK should help America prosecute the men without demanding they were spared the death penalty. 

The UK eventually agreed to waive the usual death penalty assurances in the case of El Shafee El-Sheikh and Alexanda Kotey, the Isil fighters and former British citizens who were captured in Syria.

On Thursday the High Court ruled in favour of the UK government in a legal challenge brought against the decision,…