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 world's first ever 'hole in the wall' was installed in Enfield, north London, on June 27 1967.
America says it has "identified preparations" for an attack similar to that which killed at least 86, including 30 children, in April.