NHS boss Sir David Nicholson has said "patient safety is top of the tree" and said the health service needs a "zero tolerance" of poor safety.
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An "erratic" driver sparked a security alert after ploughing into a police car near the US Congress and nearly mowing down several officers.
Thousands of people have linked hands on Westminster Bridge in a show of solidarity - one week after the terror attack in the capital.
PC Keith Palmer, 48, stabbed by Khalid Masood while on duty in the grounds of the Palace of Westminster last Wednesday.