Wales loosehead prop Gethin Jenkins believes his side face the ultimate challenge when they take on rivals England at Twickenham on Sunday.
Jenkins is hoping to claim an unprecedented third straight Six Nations title this year but knows a loss at England's home would all but end their hopes.
Wales have beaten England the last three times they have met but with Stuart Lancaster's side improving, Jenkins knows they have a real job.
"There is no bigger challenge than playing at Twickenham," Jenkins told Press Association Sport ahead of the game of the weekend.
"I've been lucky enough to get one or two wins there, but it is a fortress for them and a tough place to get a win, especially with the way England that have been performing over the last year or two and the way they have progressed.
"They are a young team and a fit team. They seem to have a very good work ethos and they play for each other. We know we have a tough battle on our hands.
"Everything is on the line on Sunday. It's pretty much whoever wins has a chance of getting the championship. If you lose, you are out of it.
"For us, having won it the last two years, we really want to make a good fist of retaining the trophy."