Mako Vunipola admits England have “no chance” of winning the Grand Slam unless there is a rapid improvement on their error-strewn performance against Italy.
An England victory over Wales in Cardiff next Saturday will complete their first RBS 6 Nations clean sweep since 2003, when Sir Clive Woodward's team conquered Europe before conquering the world.
But England's stuttering 18-11 victory over Italy has left the door open for Wales to believe they can retain their title. A seven-point victory should be enough for Rob Howley's men.
Vunipola believes England have no option but to raise their game for the Millennium Stadium decider.
"We got the win against a quality Italian side but if we perform like that next week we will have no chance," Vunipola said.
"In the changing room we all know that. It feels like we lost the game. The boys are disappointed how we performed.
"Going to Cardiff will be a tough test. We have to put in a performance that is better than this."
Vunipola spent a large part of his childhood in Wales after his father, the former Tonga captain Fe'a, moved the family over when he signed for Pontypool.
"To be involved in a Grand Slam game is amazing but we are up against a Welsh side who are on good form and we know Cardiff will be a tough test," Vunipola said.
"I've been to the Millennium Stadium once, to watch Wales against France when I was real young.
"You see the Welsh passion for rugby and there is no better atmosphere at the Millennium Stadium. It will be special for me but also for the team.
"But we have to concentrate on our performance and not let the atmosphere get to us."