Mako Vunipola insists England's training camp in Denver was so tortuous that he began reconsidering his future in rugby.

The World Cup hosts spent two weeks at altitude in Colorado last month to sharpen their conditioning for the global showpiece on home soil.

Vunipola has the opportunity to display the benefits of a gruelling summer on the training field when he starts the first of three warm-up games that sees France visit Twickenham on Saturday.

“There were a few times during the Denver training camp when I did have to question why I wanted to play this sport,” the loosehead prop said.

“You had to dig in and it was one of the hardest sessions I have known. It was a bit like torture but you had to go into survival mode and grind on. Now we are all excited because it’s the rugby time.”