Defence coach Kevin Sinfield has warned England that quarter-final opponents Fiji pose "a real threat" to their hopes of Rugby World Cup glory.

"They have some outstanding players, a massive breakdown threat, some unbelievable athletes and they play with a really good spirit about them," Sinfield said.

England's defeat by Fiji was their third in four warm-up games.

Sinfield said the subsequent debrief brought frustrations to the surface.

"We were all pretty disappointed when we reviewed it properly a few days later and pretty emotional about what had happened and about the month and our preparation and where we were at," he added.

"Although we had a lot of belief and confidence in the squad, we were disappointed it hadn't transferred on to the field.

"Fiji was the game on the screen, but I think it was a much wider message than just Fiji, it was more about us and how we go about our business and some changes we maybe need to make.

"I think I am quite honest, balanced and fair in how I coach and how I review and how I speak to players and my relationship with players, it was just an honest appraisal afterwards, whether that had an effect or not I am not sure."

Since then England have won all four of their games at the World Cup, topping Pool D to make the last eight.

Fiji edged out Australia to take the second qualifying spot from Pool C and England head coach Steve Borthwick believes that the Pacific Islanders' discipline could be as telling as their trademark offloading and big hits in Marseille.

Fiji conceded only 35 penalties in total across their four pool games compared with their opponents' 60, finishing on the right side of the penalty count on each occasion.

England, meanwhile, fell foul of referee Andrew Brace on a number of occasions in their win over Samoa on Saturday.

"It isn't often associated with Fiji, but they have been phenomenal in that area, winning lots of penalties," Borthwick told BBC Radio 5 Live.

"I am going to have to get a greater understanding of the interpretations that were applied on the pitch to certain scenarios

"We felt there were a few things there that were slightly confusing.

"That is going to be very important against Fiji because they contest the ball on the floor virtually every breakdown, so we need to understand just exactly how that is going to be officiated."

The winners of England and Fiji's quarter-final will face hosts France or holders South Africa in the semi-finals.