England face the prospect of entering their clash against Scotland without Owen Farrell after concerns over his fitness escalated on the eve of the Twickenham showdown.
Farrell was unable to take part in the captain's run on Friday due to the unspecified leg injury sustained in training 24 hours earlier and he now faces a race against time to prove he can take part.
"Owen didn't train today (Friday) 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."
England have opted against calling in another player. The most probable contingency if Farrell is forced to withdraw would see Ben Te'o promoted from the bench to fill the void and Jonny May, who is currently acting as 24th man, stepping on to the interchange bench.
England will give Farrell, who has emerged as their most important player under Jones, every opportunity to demonstrate his fitness for a match that could deliver a record-equalling 18th successive Test victory.
Only New Zealand have previously amassed that number of wins, setting the milestone last year.
England are the Six Nations' solitary unbeaten team, keeping them on course for a successful defence of the Grand Slam, yet they have not played to their potential so far in the tournament.
"We've done a lot of good stuff. In the last two weeks, not to sound too much like a broken record, but we've focused on ourselves a lot," Gustard said. "We're looking to improve.
"The boys are a in a good place, they're happy and excited to play for their country. Everyone is looking forward to playing Scotland.
"Scotland have got some good players, but then every team does."