Alexanda Kotey and El Shafee Elsheikh are said to have been part of the terrorist cell that beheaded numerous British and American citizens, including the journalist James Foley. The pair, currently detained in Syria by Kurdish forces, are likely to stand trial for these crimes in the United States. Part of the reason Guantánamo Bay remains open is that it can be extremely difficult to secure convictions in such cases; the US will want as much evidence as possible, and the UK, which has been gathering intelligence for years, will have a lot.

The UK's policy has long been that it will not share evidence unless an assurance is given that the death penalty will not be imposed. On 22 June the home secretary, Sajid Javid, wrote to the US attorney general, Jeff Sessions, to say that no assurance will be sought in this case. That may be unprecedented: the government has not yet been able to name a single instance when it had not sought such an assurance in the past.

Diane Abbott tabled an urgent question in the House of Commons yesterday. Javid did not attend; Ben Wallace, a Home Office minister, answered on his behalf. In his letter to Sessions, Javid referred to ‘strong reasons for not requiring a death penalty assurance in this specific case’. Wallace would not say what they were: he claimed, without explanation, that this would undermine the US investigation.

Perhaps officials at the Home Office did not realise how extraordinary their position was. It is, either way, hard to envisage any reason – beyond a desire to appease the US; Javid's letter referred to their 'frustration' with the case – that an assurance would not be sought. Reference was often made to the gravity of the crimes. But why is that relevant? The government opposes the death penalty, the prime minister’s spokesman affirmed, ‘in all circumstances as a matter of principle’. Wallace sometimes spoke as if the choice were between releasing the individuals without charge, and their being charged with a risk of execution. This could only be true if the Trump administration threatened to release them should the UK insist on an assurance. Wallace seemed to deny that any such threat was made.

The episode may do diplomatic harm; the UK can no longer speak without equivocation about the death penalty. It certainly does moral harm: if the individuals are executed, the UK is complicit in the machinery of death. Javid's reasons had better turn out to be very strong.