Lancaster completed the penultimate wave of cuts to his pre-tournament squad on Friday, with wing Chris Ashton the highest-profile victim among the seven trimmed, leaving 39 players to compete for the 31 places available.
England’s build-up begins in earnest when France visit Twickenham on Saturday, and Lancaster is to field his strongest team while taking the chance to examine fringe contenders such as centres Sam Burgess and Henry Slade for the remaining spots.
A second game against France in Paris seven days later will offer the final chance before the August 31 deadline for submitting World Cup squads, and Lancaster says it is these selection decisions that are proving the most vexing.
“I think the hardest bit in the build-up to this World Cup is getting from where we are to 31,” he said. “Once we have got to the final 31 then I think selection will become clear as to what is the best team to start.
“Making the decisions about who stayed [for this week], and then for who is going to play against France, is difficult. The make-up of the 23-man squad for France is definitely exercising my mind at the moment.”
England will start their strongest side against Ireland at Twickenham on September 5 knowing they must hit the ground running when the World Cup opens against Fiji 13 days later.
The presence of Australia and Wales in Pool A had already identified the group as the toughest in the tournament’s history, but the Wallabies’ coronation as 2015 Rugby Championship winners after toppling New Zealand and Fiji’s Pacific Nations Cup success have highlighted the task ahead.