Will Carling insists being prepared for the hostility that awaits in Cardiff holds the key to England completing an achievement that he believes falls only narrowly short of winning the World Cup.
The Millennium Stadium will host the decisive match of the 2013 RBS 6 Nations on Saturday, when Wales attempt to deny Stuart Lancaster's side a first Grand Slam in a decade and snatch the title for themselves.
Carling, who was at the helm for the clean sweeps in 1991, 1992 and 1995, said: "For England to win a Grand Slam in Cardiff is about as hard a task as you could get, probably not quite as hard as winning a World Cup in Australia but it's pretty close in terms of its rivalry. It will be right up there. If they manage to do it, it will be an incredible achievement."
He added: "Around 80% of England's week will be mental. Most of it will be about getting their minds ready for what they will face in terms of atmosphere at the Millennium Stadium.
"They do that by discussing what rugby means to Wales, right down to the history of Welsh rugby, and developing a respect for them.
"They must also understand the emotion of the occasion, what the crowd will be like and how passionate the players will be.
"For a lot of these guys it will be the first time they've played in Wales, so they really have to get their heads around what is waiting for them.
"As long as they're prepared for the noise, emotion and passion they'll face, they can deal with it.
"It's when you walk out and it's a shock to you that it knocks you and then you're on the back foot. England must be on the front foot from kick-off."