ClientEarth is an environmental lawyers organisation that brought the case to the Supreme Court. It comes after the Court of Appeal declined an application for judicial review of the Government's plans to improve air quality.
Today Lord Carnwath JSC backed ClientEarth's case, ruling the Government is breaching article 13 of the EU Air Quality Directive.
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David Cameron said the Met proved it can "do more with less" and declined to criticise the Mayor of London for his water cannon spend.
A group of disabled activists threw toys and balls at him as he entered the conference.
A wet start to the week and the unsettled weather is set to continue over the next few days.