Geoff Parling has claimed that England will pay the price if they are not on top of their game against Fiji on Friday evening.
Stuart Lancaster's side enter the match as firm favourites to start the World Cup with a win against a side they have never lost to in five previous meetings.
However, Parling believes that Fiji will be dangerous opposition if England are slow out of the blocks in the Pool A opener at Twickenham.
"They are much better organised and have improved immensely in the past two seasons," the Exeter Chiefs lock is quoted as saying by BBC Sport.
"If we're not on form they'll be a threat, But it's about what we can do against them."
England will also meet Wales, Australia and Uruguay in Pool A.
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