The Prison Service has refused to comment on the detail of what is claimed to have happened when Woolwich murder suspect Michael Adebolajo was assaulted in prison on Wednesday.
A spokeswoman said: "The police are investing an incident that took place at HMP Belmarsh on July 17. It would be inappropriate to comment while the investigation is ongoing."
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Through the day, most of the cloud should break up to leave fine conditions for many with spells of sunshine.
The Eiffel Tower in Paris was plunged into darkness, with the Burj Khalifa in Dubia emblazoned with the Union Jack.
The major party leaders' campaigns for the General Election will remain suspended on Wednesday following the Manchester terror attack.