England coach Stuart Lancaster admitted his team's tense 18-11 win over Italy was their worst display of this year's Six Nations Championship.
"I don't think there's any doubt about that," he said. "At half-time, we weren't quite accurate enough but still felt we were in control.
"We let that slip in the second half and Italy pushed us right to the end."
Wales could win the title by beating England, prompting Lancaster to admit: "We'll have to improve."
An eight-point victory for Wales in Cardiff on Saturday would give them the championship, but a seven-point margin could also be enough.
If the teams end with the same points difference, the title would be decided on tries scored. The Welsh have currently recorded two more than England.
The only try in Sunday's match at Twickenham came in the second half from Italy's Luke McLean.
England, who led 12-3 at half-time, needed all 18 points from the boot of Toby Flood and some resilient defending to secure their win.
"Our accuracy overall wasn't quite good enough where it needed to be," Lancaster said. "I thought we were good in a lot of areas but not our accuracy with that final pass.
"We're delighted to get the win and relieved because of the pressure they put us under in the last 15 minutes."
England skipper Chris Robshaw agreed with his coach.
"We weren't great today, but it's a win at the end of the day," he said. "We were under no illusion how hard it would be."
After scoring four tries in their opening fixture, a resounding 38-18 win over Scotland on 2 February, England have managed only one in their last three matches and again failed to capitalise against the Italians.
"There were moments when we played very well and the attack was flowing, but the final bit of execution let us down," Robshaw said. "There's a lot of work to do, going into Wales next week."
Hooker Tom Youngs added: "We gave Italy a way into the game by not taking our chances early on and we got exactly what we expected from them, a real physical encounter. We were hanging on at the end there.
"They've got some strong boys and put us under a lot of pressure, but we're glad we came through the challenge.
"We're disappointed we didn't put some chances away. I think we were a bit over-eager, got a little bit carried away, we saw the space was there and we were almost too quick to flood it. We got a bit flat and gave them opportunities.
"It's positive we created those chances in the first place - we looked dangerous early on - but once you give the Italians a way back they're going to take it. By the end, we were pretty desperate.
"We're not playing the most flamboyant rugby at the moment, but teams are raising their game for us because they know the challenge we pose. As we keep winning, the expectation gets bigger."
That will culminate in Saturday's eagerly-awaited clash with Wales at the Millennium Stadium.
"Wales are going well, they've improved week on week and they're certainly not going to roll over on their home ground," added Youngs.
"It's a huge opportunity, a huge challenge, but one we're really looking forward to."
England may have to face Wales without lock forward Joe Launchbury, who left the ground with his arm in a sling as a result of an elbow injury.
The prognosis is more promising for fellow lock Geoff Parling, who was also forced to come off the field with a shoulder injury.
Wales, who won all five games in 2012 to become Six Nations champions, began the defence of their title with a 22-30 home defeat by Ireland.
But they hit back with away wins over France, Italy and Scotland.
England won the Six Nations in 2011 but have not completed the Grand Slam since 2003, the year they won the World Cup.