A rapper from Lewisham has released a single to support the campaign to save Lewisham Hospital's Accident and Emergency unit.
The video for Save Lewisham A&E by Question includes footage of a demonstration that attracted 25,000 people.
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A Network Rail spokesman said its engineers are on site and would fix the problem "as soon as possible". Disruption is expected until 10am.
Visas will be denied for low-skilled workers earning below a certain amount and a points-based system will come into force.
At least five flights approaching Heathrow were forced to land at alternative airports due to a runway closure.