Billy Vunipola has warned England must stop France building up a head of steam at Twickenham on Sunday if his side are to continue their impressive start to the Six Nations.

Last week's opening round of matches saw England end champions Ireland's hopes of back-to-back Grand Slams with a 32-20 win in Dublin.

France, by contrast, somehow squandered a 16-0 half-time lead before losing 24-19 to Wales in Paris, their chaotic display after the interval summed up when Sebastien Vahaamahina only learned he had taken over the captaincy from referee Wayne Barnes.

Nevertheless, Vunipola was adamant France had shown enough quality in the opening 40 minutes against Wales to put England on their guard.

"What happened against Wales probably shows how dangerous they can be, more than anything," Vunipola told reporters at England's training base in Bagshot, southwest of London, on Saturday.

"What Wales showed us was how not to start a game. If you give them a leg up, then France are extremely, extremely dangerous," the Saracens back-row added.

"We've all played against French sides -- once they start feeling it emotionally then they're very, very tough to stop, so we can't allow them to get into that position."

- 'Massive blokes' -

France coach Jacques Brunel has made six changes to his side, including several among his forwards, and Vunipola, no lightweight himself, said: "They're massive blokes and if you give them momentum they're tough to stop. As a pack we must confront the challenge of their big carriers.

"They've made a few changes up front, but it's probably added more to them than taking away any of their strengths.

"They're probably a bit more athletic with Yacouba Camara in at six and Demba Bamba has some good footwork at tighthead prop. He's a big, big human being."

Victory at Dublin's Aviva Stadium gave England an encouraging start to a year that culminates with the World Cup in Japan.

But Vunipola, echoing the thoughts of England coach Eddie Jones, insisted: "The challenge is to be better. We've spoken all week about being better.

"We have leaders and coaches who aren't happy having goals that aren't the same or similar. We want to push forward and see if we can improve."

France may be familiar foes for England but they will have additional insight into Les Bleus from Scott Wisemantel, their attack coach.

Wisemantel, who had spells on the staff of several French Top 14 clubs in Clermont, Montpellier and Lyon, said: "France is a great place to work and it gives you a really good insight into the psychology of the French.

"In an away game, there is an element of freedom about them. They don't feel the pressure that they do at home."

He added: "The second thing is how they construct a game, penalties to build the score - three, six and nine, putting the pressure on the home side. Then if they are close on 60-65 minutes they can capitalise on any mistakes."

As for tales of French players smoking and drinking before games, Wisemantel said: "I have seen some crazy stuff over there!

"Their work-life balance is very different to other places. They run on emotion and you can't let that come into the game."