To prove his fitness Hartley will be put through a gruelling training session at Northampton, overseen by a member of England's back room staff, that will tell Jones if he is able to continue as captain for the looming Grand Slam defence.

"Dylan has a game simulation, so we'll have a pretty good idea how he's shaping up then," Jones said.

"He'll do a session that's the equivalent of a game. A game of Test rugby is approximately 36 minutes of ball in play. You have to run around 120 metres per minute, that's the average speed.

"It's old school - run hard, get up, get off the ground, run again. It will give us a very good indication of where Dylan is and his ability to play rugby.

"He's got to be able to do a session of that quality and if he can do a session of that quality he can do a game.

"We'll make a decision on him on Wednesday, one minute after he's done the session because we're going to name our squad on Friday.

"I don't have to worry about him not passing. I'm anticipating him going well because all his training parameters are good so far.

"It's like having a racehorse that does trials - if they run fast in the trials there's a good chance they're going to run fast in the race."

England open their Six Nations on February 4 and Jones believes Hartley's conditioning has benefited from an enforced absence that came after he was charged for striking Leinster's Sean O'Brien at Franklin's Gardens last month.

"I've never seen Dylan look fitter. You'd want him to have been playing rugby, but he's certainly benefited from six weeks of strength and conditioning," Jones said.

"That's probably put him in better condition than he was for last year's Six Nations. It's a combination between Northampton's and our staff."

England's injury profile continues to change after Jones refused to rule Joe Marler out of the opener against France, but placed a question mark over the prospect of George Kruis taking part.

Marler was expected to miss at least the first two rounds due to a fractured leg while Saracens stated last week that Kruis would recover from his fractured cheekbone in time for the visit of Guy Noves' men.

"Marler might be all right for the France game, you never know. He's down at Brighton, they have good sea air down there so the recuperation can be quicker," Jones said.

"Kruis is going to be borderline for the French game. It's just the way the cheekbone is.

"You can have a broken cheekbone and play the next day or you can have an operation and it will be three or four weeks. It's just how they mend."