Health Secretary Jeremy Hunt has had his decision to reduce services at a major hospital declared unlawful and quashed by the High Court.
There was loud clapping in court as a judge ruled that Mr Hunt acted outside his powers when he announced to Parliament in January that casualty and maternity units at Lewisham Hospital in south-east London would be downgraded.
Mr Justice Silber said the Secretary of State had breached provisions of the National Health Services Act 2006.
The ruling was a victory for the London Borough of Lewisham and the Save Lewisham Hospital Campaign, a community-based campaign group made up of and supported by patients, community groups, GPs, hospital doctors, nurses and other health professionals.
Elisabeth Laing QC, for the council, told the judge at a recent hearing at London's High Court that Lewisham Hospital was a "very good hospital" and the range of health services provided there was "greatly valued by local people and by the council".
If the changes were made, it would mean local people "will have to travel a long, long way further to get access to vital services", said Ms Laing.
The proposed changes were part of a wider shake-up of services in the capital after the financial collapse of neighbouring South London Healthcare NHS Trust (SLHT), which went into administration after it started losing more than #1 million a week.
Mr Hunt told MPs the changes would improve patient care in south London, saving up to 100 lives a year, but gave an undertaking not to implement them pending today's legal challenge.
Dr Louise Irvine, a local GP and chair of the Save Lewisham Hospital campaign, said:
"This is an incredible day. We are delighted for every single person who has supported the campaign and those who will now continue to benefit from this extraordinary hospital.
The support from thousands of people in Lewisham is a very real demonstration of the Big Society.
David Cameron himself said there would be no 'top-down' approach to closures and we appreciate the court's decision, which should serve as a reminder to this Government not to forget their promises and not to underestimate those who they seek to represent."
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