Boris Johnson warned that helping the US prosecute two alleged Isil executioners would prevent Britain demanding others are spared the death penalty in future, The Telegraph can reveal.

The then foreign secretary expressed reservations as he and Home Secretary Sajid Javid discussed waiving death penalty assurances in order to help US prosecutors bring the two men, part of an alleged Isil terror cell nicknamed "the Beatles" to trial.

He ultimately agreed that "the benefits outweigh the risks" of the two men potentially escaping justice and being free to return to the UK, but his concerns - detailed in a “sensitive” briefing document circulated to key civil servants and ministers - are likely to be jumped on by critics who believe the change of policy undermines the UK’s opposition to the death penalty in the long-run.

This newspaper revealed on Monday that the UK had agreed to hand over intelligence on the jihadists Alexanda Kotey and Shafee El-Sheikh to US prosecutors, but dropped demands they avoid execution.

Both men are suspected of being part of the ‘Beatles’ terror cell who were behind the beheadings of two British aid workers and three Americans, plus scores of others.