Planned cuts by police forces fall £500 million short of the savings needed, Whitehall's spending watchdog has warned.
The National Audit Office (NAO) said two-thirds of police forces across England and Wales have not made enough cuts and will not meet their savings targets.
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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.