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."
More top news
An eruption in June killed 194 people and left at least 234 missing.
Jeremy Hunt has warned Iran against using innocent dual-national Britons as political prisoners as he prepared to visit the country.
Skies will be on the cloudy side today, although there should be some sunny spells too.