England have just one more warm-up match before travelling to New Zealand, against Ireland next Saturday, and there has been plenty to concern manager Martin Johnson about their recent displays.
They had looked in their best shape since their 2003 World Cup triumph during last year's autumn internationals and the start of this year's RBS 6 Nations.
But performances tailed off and they failed to complete the Grand Slam, while last weekend's defeat in Wales seemed to suggest they had taken a step back.
Leonard, who is still Test rugby's most capped forward, insists England only need to be the best side on the planet for four weeks to win the World Cup.
And he challenged them to show they could eke out victories like their 2003 counterparts did in Australia.
"They have got to go out and treat each game as their last," said Leonard while promoting the second Help for Heroes benefit match, the 'Heroes Rugby Challenge', which takes place at Twickenham on December 3.
"In a World Cup, it doesn't matter if you win each game by one point and get through to the final and win by one point.
"You don't have to be the best team in the world - which I think is how New Zealand have approached it - for four years.
"You only have to be the best team in the world for four weeks and you've won a World Cup."
Johnson will unveil his 30-man squad for the tournament on Monday and reports suggest Harlequins winger Ugo Monye and flanker Chris Robshaw, and Saracens lock Mouritz Botha, will be among those to miss out.
It also looks like Lewis Moody will be named captain, providing his troublesome right knee injury does not flare up again, even though the flanker may not feature against Ireland in Dublin next weekend.
Moody would lead a pack good enough to topple the world's best, according to Leonard, who urged the back line to take advantage of their good work.
"I think the English pack could take on the All Blacks tomorrow," he said.
"We've got to now develop that to our 15-a-side game as well.
"If we're not going to convert our chances behind the scrum, you're always going to be in trouble."
He added: "The England team of old, we used to keep the scoreboard ticking.
"We'd always be trying to put scores on the scoreboard."
To that end, Leonard has backed 2003 hero Jonny Wilkinson to start in the number 10 jersey in New Zealand, with Toby Flood having suffered a dip in form since the start of the Six Nations.
"We all know what Jonny can do," Leonard said.
"Even if Jonny didn't play a game up until the World Cup, Jonny would walk out onto a pitch anywhere and he'd be good for 15 points, no matter what.
"He's world-class and he's got that written all over him.
"So you know what you get with Jonny.
"I suppose what they're trying to find out with the Flood factor at the moment is, 'What do we get with Toby?'.
"I think we got flashes of it last year when he played really well in the Six Nations and the autumn internationals.
"But he didn't really seem to hit the ground running on Saturday.
"Whether he gets another chance, we'll see."
For the first time in World Cup history, England are in a pool they could conceivably fail to qualify from, with Argentina and Scotland both blocking their path.
Leonard said: "Scotland would love the opportunity to put one over on England, as they have done over the last couple of years.
"I don't think anyone has an easy game with Argentina, and in World Cups they're very good at upsetting people as well.
"That just shows there cannot be any relaxed form of play from England."
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