Boxing promoter and Dereck Chisora manager Frank Warren has said there is "nothing illegal" about the fight sanctioned by the Luxembourg Boxing Federation during a press conference at West Ham's Upton Park.
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Cloud and outbreaks of rain will ease away eastwards during the morning, clearing the east coast around midday with sunny spells developing.
Police and emergency services are attending the scene.
Rebel-held east remains at risk while the pumping station supplying the western part of the city is once again in operation.