Nick Clegg and Nigel Farage have clashed over British foreign policy regarding Ukraine, Russia and Syria.
Mr Farage also attempted to clarify comments he made about Russian president Vladimir Putin, saying he admired him "as an operator" but not as a man.
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Among those murdered was Keith Palmer, an unarmed constable who "did his job and never wanted any fuss."
Top temperature 14 Celsius (57 F).
The Prime Minister will say Moscow's brazen flouting of international law represents a threat to the basis of democracy across Europe.