MI6 chief Sir John Sawers said the leaks from NSA whistleblower Edward Snowden "have been damaging and put operations at risk."
Al-Qaeda is "lapping up" the leaks and Britain's enemies are rubbing their hands with glee, the British intelligence chief added.
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A band of heavy, thundery showers continues to move northwards through this evening, dying away overnight.
The Duke of Cambridge also laid a wreath to honour the victims before kick-off.
The mother of one of the Manchester attack victims has told of the distressing moment she found her daughter being treated after the attack.