Mobile phones or tablets with flat batteries will not be allowed on flights as part of new security restrictions. All air passengers flying into or out of UK airports must ensure their electronic devices are charged, following updated guidance from the Department for Transport (DfT).  Advice issued at the weekend concerned passengers flying to the United States, but on Tuesday that instruction was widened to affect anyone flying into or out of the UK.  No specific routes have been highlighted, and the DfT spokesman said they hope to ‘minimise disruption as far as possible’.  The heightened security comes amid reports that two terror networks are working together on a bomb that could evade existing measures.   Last week Prime Minister David Cameron said decisions had been made based on ‘the evidence in front of us’ and in co-operation with America.  ‘The safety of the travelling public must come first. We mustn't take any risks with that,’ he told the BBC.