British counter-terrorism police said today they had launched an investigation into an Islamic State video purporting to show the beheading of American journalist James Foley and featuring the voice of a man with a British accent.

The Metropolitan Police Service Counter Terrorism Command, that aims to protect London and the UK from the threat of terrorism, said in a statement that it was investigating the contents of the video that was posted online.

"We would like to remind the public that viewing, downloading or disseminating extremist material within the UK may constitute an offence under Terrorism legislation," the specialist operations unit added in the statement.

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