Mandelson made the announcement on June 15.
The move, which is due to go into effect on June 21, will increase the cost of some Belarusian goods to Europe, mostly farm machinery and chemicals.
It will not affect oil and gas exports from Russia that pass through Belarus to Europe.
The Geneva-based International Labor Organization had announced earlier on June 15 that Belarus had failed to make progress on giving workers the right to join unions.
The European Commission said the trade preferences will be restored as soon as Belarus complies with International Labor Organization standards.