Owen Farrell a doubt for England's Calcutta Cup clash against Scotland
England face the prospect of entering their Calcutta Cup 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 in the Triple Crown decider.
The Saracens playmaker, due to start at inside centre, faces a period of intensive rehabilitation in his quest to be ready, with England prepared to make a late call.
"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. We'll make a call as and when we know more."
England have opted against summoning any fly-half or centre cover, instead relying on the 24 players retained for the penultimate round of the RBS 6 Nations.
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 replacements bench.
However, if fly-half George Ford were to be struck by injury the reigning Six Nations champions would be left exposed at fly-half as beyond Ford and Farrell there is no established alternative in the matchday 23.
Alex Lozowski is the next option on the pecking order but he has been named on the bench for Saracens' Anglo-Welsh Cup semi-final against Leicester on Saturday evening, and England have already insisted he will not be involved at Twickenham.
"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 (of the 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."
Uncertainty over Farrell's injury has risen since a Eddie Jones on Thursday gave conflicting answers about his availability to face Scotland.
Jones joked that the Saracens playmaker initially sustained the injury - the details of which England will not divulge - during a collision with his dog Annie during the final training session of the week at the squad's Surrey base.
When asked for an update on Farrell, a prickly Jones offered contrary messages.
"Owen could be a doubt, he could be a doubt. He's got a bad leg, so he couldn't finish training. He'll be all right,'' Jones said.
Farrell was expected to be given the green light, but his inability to take part in the session at Twickenham on Friday has set alarm bells ringing.
Alongside Ford and Mike Brown, the world player of the year nominee was involved in kicking practise overseen by Jonny Wilkinson but would only rehearse technique rather than actually putting boot to ball.
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. We're looking to improve," Gustard said.
"We have objectives that we're looking for and I think we're in a good place. The mood is exceptional.
"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."