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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163,000 French and 143,000 German soldiers were killed in the 10-month battle in 1916.
As many as 700 migrants have been died after three consecutive days of shipwrecks off the coast of Libya, the UN Refugee Agency has said.
The fire broke out on Sunday morning at the home for 35 residents in Litochki, a village 25 miles north of Kiev.