Lawrence Dallaglio says England must not get carried away by their 62-5 win over Wales because they face a much harder test against France on Saturday.
The game is England's last before coach Brian Ashton has to name his 30-man World Cup squad on 14 August.
"It was encouraging but we've all been around the squad long enough to give ourselves a dose of reality," he said.
"France are in a different situation, they've named their squad so whatever team they put out will be world class."
When Dallaglio runs out at Twickenham on Saturday it will be his first Test start for England since 2004.
"I know everyone is looking forward to the World Cup, but as someone who's been out for three years, playing against France is the first part for me," he said.
"We'll have to be more flexible and subtle, so I'd like to think the backs will come into play a lot more "
"I see it as a three stage process; these warm-up games, then the World Cup group stage games and then hopefully the latter stages.
"Let's not think about stages two and three before we've done one - we made a good start last week, we know this is a big step up in class and we hope we'll be up to the challenge."
In the win over Wales, number eight Nick Easter scored four tries, Dallaglio bagged one, and lock Steve Borthwick also got in on the act.
But Dallaglio said this did not mean England were resorting to a totally forward-dominated style of play.
"In the last few years we've moved away from the fundamentals that made England strong, and the game against Wales was about putting some of those basics back into play," he said.
"Against better opposition, we're not going to dominate possession and territory as much, and we'll have to be more flexible and subtle, so I'd like to think the backs will come into play a lot more - but don't forget that style of rugby comes from confidence and momentum.
"I think the combination of forwards and backs will evolve - whether it does in time for the World Cup remains to be seen.
"We'll do our best, but the first priority is to win the game by whatever means, and that was certainly achieved at the weekend."
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