George Osborne has said Moody's message is "explicit" to carry on with austerity.
He says "we can either abandon our efforts to deal with debts" or we can "redouble our efforts" and make a brighter future for our children.
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The US president is seeking an investigation by the Justice Department.
The casualty was taken into the care of paramedics with a suspected leg injury.
The 16-year-old was discovered with stab wounds on Thursday afternoon.