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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Vehicle imports and exports could be seriously affected by a no-deal Brexit scenario.
Theresa May has told the three million EU citizens living in Britain she will make it as easy as possible for them to stay after Brexit.
Ahead of its roll-out in Manchester, some Universal Credit recipients describe feeling "trapped" as they wait weeks to receive benefits.