David Cameron's plea for "responsible capitalism" is failing because he wants to do it by asking businesses to behave better, rather than changing the rules, Ed Miliband said.
The Labour leader highlighted Cameron's famous 2006 attack on WH Smith for stocking chocolate oranges rather than real oranges by the tills, which he said had no impact on retailers' behaviour.
The prime minister's decision to appeal for moral capitalism in a high-profile speech last week was a sign that Labour was "shaping the battle of ideas" after he identified the issue in his conference speech four months ago, said Miliband.
The Labour leader told The House magazine that, six years after Cameron criticised point-of-sale discounts on chocolate oranges, the tactic was still being used by retailers.
"If he can't sort out the chocolate orange, he's not going to be sort out the train companies, the energy companies, the banks, is he?" Miliband said.
"I think it's very interesting that David Cameron's example of responsible capitalism was the chocolate orange.
"He's failed to sort it out. Why? Because of his basic set of beliefs.
"You know, he believes in a 'nudge' philosophy which seems to amount to just asking people to do nice things. But that isn't going to sort out the problem. You've got to change the rules."
Miliband said he faced a lot of criticism for taking on "predatory capitalism" in his conference speech last September.
But he added: "Three or four months later, (Nick) Clegg and Cameron are saying 'We're sort of on the same page'.
"That tells you something about them knowing that I'm making the right arguments. Labour is shaping the battle of ideas, and that is really important because I don't think we were doing it at the last election."
Miliband also said he believed Labour's message on the economy was getting through to ordinary voters, who were beginning to quote back to him his analysis that the Government was cutting "too far and too fast".
He defended his controversial decision to back the Government's public sector pay squeeze, which provoked what he suggested was a "watershed" row with unions.
Referring to deficit cuts, he said: "I think the Government has actually started to lose that argument. I think though our task is now to win the argument and to show that we can be credible with the nation's finances and that is a really important task for us.
"I think we were taking, and are taking, the right position... the position that says, if it's a choice between jobs and the scope for big pay rises in the public sector or in the private sector, the choice should be jobs.
"I realise that's really tough and difficult for people. But, you know, Labour Party leaders and trade union leaders go through their watershed moments quite often it seems to me."
He dismissed suggestions that unions might disaffiliate from Labour. And he rejected the idea that his leadership was in trouble because Labour had not established a clear advantage in the polls.
"The issue for an opposition at this stage of a Parliament is 'Are you doing the right things? Are you winning the battle of ideas?' - which I think we are, with responsible capitalism," he said.
"The race is probably one third run. Let's see where we are at the end of the race."
Polls were "one of those things", said Mr Miliband. And he said he did not "really" follow newspapers or TV news, preferring to spend time with his children when he gets home.
Miliband said he did not agree with former prime minister Tony Blair that it was a mistake to axe the Royal Yacht Britannia, and insisted that a replacement was "not a priority for public money".
Asked if he would donate to a privately-funded Royal Yacht, he replied: "I give money to charity in different ways. There are obviously lots of deserving causes."
The interview prompted Labour Lord and sometime social media guru John Prescott to start a #cameronchocolates hash tag. See below for a selection of the best tweets.