LONDON (REUTERS) - Britain is looking for a"proportionate way" to respond to the threat posed by Russia, a spokesman for Prime Minister Theresa May said on Tuesday (Apr 3), after a retired Russian army official said the poisoning of a former spy could start a new world war.

"We need to respond in a proportionate way to this aggressive behaviour from Russia and that's what we're doing,"the spokesman said when asked if there was a real risk of triggering a war.

Evgeny Buzhinsky, a retired lieutenant general, was quoted in British newspapers on Tuesday as saying the fallout of the attack could trigger "the last war in the history of mankind".

This story is developing.