Capital punishment has not proved to be a deterrent, 'and any miscarriage of justice - which is inevitable in any legal system - is irreversible', the statement said.

The EU, which has abolished the death penalty, renewed its call on US federal and state authorities to introduce 'a moratorium' on further executions.

Capital punishment is currently legal in 35 US states, but most already have moratoriums or use it extremely rarely. The vast majority of executions occur in only four states - Texas, Virginia, Oklahoma and Missouri.