Shadow chancellor Ed Balls has said the downgrading of Britain's credit rating is in the Chancellor's own words, a "humiliation for this government".
Mr Balls said George Osborne had failed the first economic test he set himself.
"He used to say downgrade would be a disaster, now he says it doesn't matter", he added.
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The Office for National Statistics said gross domestic product grew by 0.1%.
William's comments came during a surprise appearance at an event scheduled for just his brother.
Matt Campbell, who collapsed just a few miles from the finish line, had hoped to raise £2,500.