England coach Brian Ashton was at a loss to explain his side's dismal display in their World Cup opener against the USA in Lens.
The last 20 minutes were very disappointing from our point of view
England coach Brian Ashton
The defending world champions ground their way to an uninspiring 28-10 win over a side ranked 15th in the world.
"We've made a winning start but it was less than a satisfactory performance," admitted Ashton.
"There was a lack of physicality and energy but I've no idea why - I can't put my finger on it."
England scored three tries through Jason Robinson, Olly Barkley and Tom Rees, but a high error count undermined any chance of Ashton's side following the lead set by New Zealand and Australia earlier in the day.
The All Blacks and the Wallabies powered to ruthless wins over inferior teams to set out their stall for the tournament, but England's display is unlikely to have worried any of their main rivals.
"I thought we'd kick on after we scored the third try but suddenly we started having problems at the line-out and you can't build your game," added Ashton.
"The last 20 minutes were very disappointing from our point of view, without taking anything away from the USA who hung on until the end."
England take on South Africa in Paris on Friday, and they will need a massive upturn in form to pose any sort of threat to the Springboks.
Captain Phil Vickery also admitted his side's showing, coming on the back of successive losses to France in the warm-up matches, had been substandard.
"I am delighted with the win but disappointed with the performance," said Vickery.
We are not going to over-react but we know we didn't play very well
England captain Phil Vickery
"The USA made it made it difficult as we knew they would.
"What was it down to? Nerves, opening game, a lot of hype in the papers but you've got to be a little careful as emotions do run high after a match.
"We are not going to over-react but we know we didn't play very well."
Vickery also insisted his trip on centre Paul Emerick, which denied the USA an almost certain try, was not deliberate.
"It was unfortunate," he said. "I was back-tracking like a good prop forward, always looking for a corner flag, and unfortunately I tapped him.
"But there was certainly no malice from my part. I thought the game was played in a good spirit and I am glad there was no damage done to him or myself."
Vickery said his trip on Eagles centre Emerick was not malicious
Emerick was also philosophical about the incident.
"The referee was behind the play and maybe he will look back on it and say 'maybe I missed a call there'," he told BBC Sport.
"But hands go one way and legs go another, that is just the way the game goes. I am sure it was a reaction, rather than pre-meditated."
Emerick, one of only five fully professional players in the US starting line-up, insisted he was not surprised at how close they had come to the world champions.
"I think we could have done more, and it could have been a hell of a lot closer, maybe even an upset," he added.
"We weren't going to roll over. We were the underdogs but we watched the France-Argentina game and knew we had to have a similar game plan, tackling low.
"I wasn't really surprised we got that close to them. That gives us good confidence in our defence for our next game against Tonga but we are not going to get too excited about this performance."