Owen Farrell faces a race against time to prove his fitness as fears grow over his availability for England's Calcutta Cup clash with Scotland at Twickenham on Saturday.
Farrell missed the eve-of-match captain's run because of an unspecified leg injury incurred during training on Thursday and will be given the next 24 hours to demonstrate he is ready for the Triple Crown decider.
"Owen didn't train today and we're monitoring his leg injury. It's a leg injury," defence coach Paul Gustard said. "We have until tomorrow before kick-off to make a decision, which means he has at least another 24 hours of recovery ahead of him. We'll make a call as and when we know more."
England have decided against calling in anyone as cover, instead relying on the 24 players retained for the penultimate round of the Six Nations. The most likely contingency would see Ben Te'o promoted from the bench to fill the void at inside centre and Jonny May, who is currently acting as 24th man, stepping on to the replacements bench.
However, if fly-half George Ford were to be struck by injury, the lack of cover in the position would leave England exposed.
"We're not bringing anyone in. We're obviously very happy with Ben in the squad. We have Jonny May with us at the moment as 24th man, so we have some flexibility as cover," Gustard said. "We're really confident (with our options if Owen misses out). We're very happy with the squad that we've got and the 24 that we chose for this game. Whatever combination we end up with at 10, 12, 13, we're happy."