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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The remaining X Factor contestants face the judges tonight - with an exit predicted for at least one member of boy band Stereo Kicks.
Jack Bruce, the composer, singer and multi-instrumentalist, died today of liver disease at the age of 71.
Police have confirmed that a father killed in a blaze at his home with his nine-year-old son died due to "the effects of the fire".