England centre Billy Twelvetrees is relishing a clash with Wales' Jamie Roberts after declaring he is expecting the most physical battle he has faced in Sunday's crucial Six Nations encounter.

Roberts will spearhead Wales' assault at Twickenham in a side featuring two changes to the one that beat France 27-6 with Scarlets centre Jonathan Davies and lock Alun Wyn Jones coming into the side.

Twelvetrees has faced several of the game's most feared centres in Ma'a Nonu, Wesley Fofana and Brian O'Driscoll in a developing Test career spanning 11 caps.

But the 25-year-old Gloucester centre views fellow British and Irish Lion Roberts as offering the most muscular confrontation of all.

"Yes, in terms of physicality Jamie Roberts will be the biggest challenge I have faced," Twelvetrees said.

"He's a top-class player and he's shown that in the years he has played for Wales and for the Lions.

"He's always making breaks and getting over the gain line.

"He's one of these players you look up to and he's such a good professional as well."

Meanwhile, Wales coach Warren Gatland admitted yesterday he has been impressed by England, who are level on points with Wales after winning two and losing one of the Six Nations games, but believes Wales' greater experience could prove to be a key factor at Twickenham.

"I think England are improving," said Gatland.

"I look at their side at the moment and they're missing some key players - Dan Cole, Alex Corbisiero, Geoff Parling, Manu Tuilagi if he doesn't start.

"If you look at the two sides, there's a massive amount more experience in our side compared to England's.

"They're a team on the up and they're improving as well.

"I think they're missing a number of important players who, if they come back in the next 12 months, will make them stronger given the younger players they've got at the moment will gain some experience."

Rhys Webb remains at scrum-half ahead of Mike Phillips for Wales, who is on the bench, and George North reverts to the wing with Davies coming back into midfield.