Shadow health secretary Andy Burnham gave Labour until next spring to spell out its ideas in a way that “captures how people are feeling and thinking”.
The frontbencher, who stood against Ed Miliband for the party leadership, claimed many voters had “decided the coalition is a failure as a government”, but “what they aren’t yet convinced is that we have the answers”.
In an interview with the Guardian, Mr Burnham said: “I think there’s definitely a need to shout louder, and speak in a way that captures how people are feeling and thinking. There’s definitely a need to put our cards on the table.”
Asked how long that window was, he replied not “much beyond next spring”.
Mr Burnham’s comments follow criticism of Mr Miliband’s leadership from backbench MPs, but the shadow health secretary insisted he had the “utmost respect” for his leader.
Veteran backbencher George Mudie has claimed that the party appeared “hesitant” and “confused” because of Mr Miliband’s failure to spell out a clear agenda to voters.
Mr Burnham denied he was echoing Mr Mudie’s concerns and said: “No, this isn’t a criticism of Ed ... I’m making it a criticism of modern politics.”
He confirmed there were difficulties within the shadow cabinet over his radical plans to integrate social care into the NHS, but claimed he wanted to deliver a policy for Mr Miliband which would “knock the others off the pitch”.
The Guardian reported that the proposal faced strong resistance from shadow chancellor Ed Balls, and Mr Miliband remained undecided.
“If we were to make that shift to fully bring social care within the NHS, I think we could genuinely set the NHS up for the 21st century,” Mr Burnham said.
“I’m saying to Ed [Miliband], I will give you an NHS policy that is one nation to its core, people will say that’s what one nation means, all people covered for all of their care needs in a system that is based on the values of the NHS.
“What better way for Labour to say it’s relevant to the 21st century than to bring forward a policy as bold in this century of the ageing society as the NHS was in the last?
“That’s the way that Ed Miliband wins, by having policies that just knock the others off the pitch basically. And that’s what I want to give him.”
Mr Burnham, who was health secretary under Gordon Brown, criticised the way in which Labour had opened up the NHS to private sector firms.
“Once the market takes a hold on the system it will destroy what’s precious about it,” he said. “We had been building a policy that had been saying it doesn’t matter who provides healthcare as long as it’s free at the point of delivery. But I’m saying it does matter.”
He also criticised the political culture that had developed in Westminster: “We’re the professional politician generation, aren’t we?
“I was schooled in this, kind of, how do we make a press release today that embarrasses the opposition? That’s the kind of politics that everyone was doing, and the kind of culture developed where you’re scrabbling over a bit of the centre ground with micro-policies that are designed to just create a little couple of days’ headlines and create a feeling, but not change much else.”
Mr Burnham’s comments follow interventions in recent weeks by Mr Mudie and his fellow backbenchers Geraint Davies and Graham Stringer.
Mr Davies wrote in The Independent earlier this week that Mr Miliband needs to provide a “compelling case” to the electorate on why the country would be better off under Labour and come up with a more effective response to Conservative attacks.
The party looked like a “shamefaced schoolboy” by not rebutting Tory accusations that Labour was responsible for the economic mess, he added.
Mr Stringer said the party was making “a huge mistake” by “slumbering” during Westminster’s summer recess, rather than using the opportunity to attack the Conservatives.
Conservative chairman Grant Shapps said: “This shows that even senior members of Labour’s top team think Ed Miliband doesn’t have what it takes to stand up for hardworking people.
“His lack of leadership and his lack of ideas shows he’s a weak leader of a party that just hasn’t changed. It’s the same old Labour Party and hardworking people would pay the price of its failure to face up to the mess it created.”
A spokesman for Mr Burnham added: “The Tories are using Crosby inspired tactics to divert attention from their failures on A&E, the loss of almost 5,000 nursing posts and increasing waiting times, which show that David Cameron cannot be trusted with the NHS.
“Andy is part of a One Nation Labour team led by Ed Miliband that is setting out bold plans including for whole person care, energy market reform, fairer taxes and a focus on the forgotten 50% of children that don’t go to university.”