Amnesty International have provided photographic and witness evidence that between 2011 and 2015 Syrian president Bashar al-Assad, 51, authorised the torture and execution by hanging of up to 13,000 civilian political prisoners.

Torture condoned by Assad to punish opponents and crush dissent included beatings, burnings with acid and electricity and starvation, before victims were taken to a basement and hanged in batches of 50. Bodies were then taken to hospitals where doctors were forced, under threat of death, to issue death certificates stating the victims had died of natural causes.

Britain’s foreign secretary Boris Johnson said he was sickened by the Amnesty evidence, and that Assad “has no future” as the leader of Syria.