A landmark judicial review of police practices at last year's Royal Wedding will begin later.
Measures examined will include the use of 'pre-crime' arrests - which were employed before the wedding.
The findings will likely impact future policing at upcoming events such as the Jubilee and Olympics.
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Dozens of women gathered on Westminster Bridge to form a human chain in a show of solidarity with the victims of Wednesday's terror attack.
In a wide-ranging Peston on Sunday interview, he said Parliament security needs reviewing and the PM's counter-terror strategy is flawed.
The terror attack carried out by Khalid Masood at Westminster that left four people dead lasted less than two minutes say police.