Passengers were forced to join long queues at Heathrow Terminal 5 as a technical problem at an air traffic control centre continued to cause chaos at airports across the UK and Ireland.
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The annual event has earned a place in the record books ahead of the grand final of the 60th anniversary show in Austria this evening.
Local authorities said the move is necessary to "avert the growing number of deaths and increasing harm linked to psychoactive substances".
Some students argued that the centuries-old tradition of wearing gowns, suits and mortarboards to exams was 'archaic' and looked elitist.