Despite extending their unbeaten run with a 18-11 win over Italy on Sunday, the Red Rose failed to register a single try despite being hot favourites going into the match, and came under immense pressure in the second half.
Lancaster's side travel to Cardiff on Saturday for a title showdown with fellow challengers Wales, and he admits that England simply have to do better.
Asked if England's performance against the Azzurri was their worst of the tournament, Lancaster told Sky Sports News: "Yes, I think so, certainly the second half.
"In the first half we weren't quite accurate enough but we still felt in control of the game. But I felt we let that control slip and Italy pushed us right to the end.
"We have to improve on that performance and it will be a massive step for us. We got the win today but it's a quiet changing room and we'll take the lessons out of this. It's set up for a fantastic occasion."
England's last Grand Slam Six Nations title came in 2003.