Maina Kiai, the UN special rapporteur on the rights to freedom of peaceful assembly and of association, said he was "appalled" to hear about the blacklisting of union members in the UK construction industry.
Speaking on a trip to London, he said: "It is crucial that strong actions be taken against the making and using of such lists as a deterrence."
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Police have revealed a pair of sandals they have had made similar to the pair Ben is thought to have been wearing when he disappeared
The hospital was last attacked just three days ago, something that UN Secretary General Ban Ki Moon denounced as a war crime.
Former UKIP leader Nigel Farage has flown to the US to help coach presidential candidate Donald Trump ahead of his next debate.