England head coach Stuart Lancaster will reportedly reduce his 50-man squad ahead of their departure to Denver in the USA.
Five players are expected to have their World Cup hopes ended early.
Sam Burgess, Maro Itoje, Luke Cowan-Dickie and Henry Slade are all uncapped but believed to still be in contention.
However the likes of Stephen Myler and possibly Nick Easter could miss out.
England winger Jonny May described the fitness training the squad have done as some of the toughest he had experienced in his career so far.
"The running sessions have been up there with the toughest I've done," he said.
"The training that we've done will prepare us well. We've had some really tough fitness sessions, but I'm sure it's going to be a step up in Denver.
"It will be hot and at altitude, and I think we are planning on introducing a little bit more rugby once we get out there.
"It has been very much strength and conditioning-based so far, with a bit of skills."