The former Scarlets star suffered an ankle injury on the opening weekend of Championship action.
It was hoped he would return to action to face Italy but he failed to recover and will also miss the Grand Slam clash with title-chasing Wales.
”Ben will stay at Gloucester this week. We knew his ankle would take six to eight weeks and he won’t make it in time,” said Lancaster.
England are also sweating on the fitness of locks Joe Launchbury, Geoff Parling and Courtney Lawes but Lancaster is hopeful they will recover from their respective elbow, shoulder and arm injuries.
England were given a fright against the Azzurri at Twickenham and Lancaster admits it was not ideal preparation for the encounter with Wales.
Rob Howley’s men can deny England a first Grand Slam in a decade with a victory and a win by seven points or more should be enough to defend the Championship crown.
Lancaster expects a fierce reception at the Millennium Stadium and reckons it will also provide invaluable experience ahead of the 2015 World Cup.
“We have had a chance to review the game and there were lots of areas we were pleased with, but I wouldn’t call it a blessing in disguise,” added Lancaster.
“I’m not of the mindset as a coach that I’d want to not get areas right in order to get ourselves in the right place to play a game. We’ve definitely got plenty to work on.
“Saturday’s a massive game because of the context of the result, which is huge for both teams.
“We’re a young team and a lot of our players haven’t played there before, so going to that type of environment is exactly the test we want.
“It’s a couple of years until 2015 and we know that we’ve got Wales in what will be a big high-pressure game.
“For a team to get that type of experience now and go up there and try to win a Grand Slam will be a special occasion.”
