* England on course for grand slam after 18-11 win
* Flood kicks 18 points
* Italy score game's only try (Adds details, quotes)
By Mitch Phillips
LONDON, March 10 (Reuters) - England remain on course for their first grand slam since 2003 and their second Six Nations title in three years but will have to find a rapid improvement from a ragged 18-11 home victory over Italy on Sunday if they are achieve either next week
Flyhalf Toby Flood, playing instead of the injured Owen Farrell, scored England's points as he landed all six penalty attempts but Italy played most of the rugby and completely dominated the second half, scoring the only try through winger Luke McLean.
It was hardly the warmup England wanted for next Saturday's showdown in Cardiff, where Wales, who beat Scotland 28-18 on Saturday for their third tournament win, would retain their title if they won by eight points.
"We're not punching the air, we know we need to do better," England coach Stuart Lancaster told reporters.
"We know we have to improve areas of our performance. We created opportunities but didn't take them.
"But there was some good stuff too so we're not overly depressed and overall I'm delighted to be sitting here with four wins out of four."
England actually started full of running on a freezing day at Twickenham but they struggled to turn that early dominance into points and lost their way.
Four Flood penalties to one by Luciano Orquera put the hosts 12-3 ahead at the break with the home crowd ruing three excellent try-scoring opportunities wasted by poor execution or decision-making.
Italy came out firing in the second half but were gifted their try as a horribly-sliced clearance kick by Danny Care went straight to an unchallenged Alessandro Zanni and Orquera then chipped wide for McLean to score in the corner.
Orquera missed the conversion and a penalty soon after but Italy kept their spirits up and, instead of fading in the final quarter as they have so often before, they stepped up their game and took charge.
Showing some crisp passing and real aggression at the breakdown, the Italians poured forward and forced England to defend their own line for long spells while the hot favourites barely venturing over the halfway line.
Their defence was organised and disciplined, however, to prevent what could have been a match-drawing try and leave the fifth-placed Italians frustrated at being unable to add to the two points gained from their opening victory over France.
"Our pack had a real scare, we'd had injuries and we were hanging on in their," said England forwards coach Graham Rowntree.
"It was a massive relief so credit to the lads for keeping their composure and coming through that."
Lancaster too was impressed by his side's defence and said he was already excited about the Cardiff decider.
"In some ways from a coaching point of view it's better that you have something to go at," he said after England failed to score a try for the first time in 19 matches against Italy.
"But our good attacking shape was there, our discipline was good - you could see that five minutes from time when we were defending our line.
"We know Wales are playing well but we know what we need to do and that we just need to be more composed in key areas, so it will be great to see where we are next week."
Two years ago Martin Johnson's England travelled to Dublin eyeing a grand slam but were well beaten by Ireland.
They were still crowned champions for the first time in eight years but there was a hollow feel to their celebrations and the current crop will be desperate to avoid the same scenario, or worse.
A seven-point win for Wales would leave the teams level on points difference and see the championship decided on tries scored.
Wales have scored seven to England's five in the first four rounds. If the two teams end level on tries too, the title would be shared.
Italy, who had lost all their previous Six Nations games at Twickenham by an average of almost 35 points, can take heart from their improved showing despite falling to their third championship defeat in a row.
"We're on the right road," said coach Jacques Brunel. "Each week we identify something to work on but we played well for long periods today.
"We played a lot of good rugby but paid the price for the penalties we gave away."
(Editing by Ed Osmond; email@example.com; +44 20 7542 7933; Reuters Messaging:; firstname.lastname@example.org)