Ed Miliband has insisted he can "defy the odds" and win next year's general election, despite a poll suggesting that as many as 60% of voters think he is not up to the job of being prime minister.
Mr Miliband acknowledged he had a "tough fight" to return Labour to power after one term in opposition, but said he was "relishing" the battle over the next 10 months leading up to the May 2015 poll.
The Labour leader's comments came as he launched welfare reform proposals which would see u nemployed youngsters stripped of state handouts unless they agree to training in vital skills.
Bidding to counter Tory jibes that Labour is the "party of welfare", Mr Miliband called for 18 to 21-year-olds to be given a "youth allowance" rather than out-of-work benefits.
The money would be conditional on them signing up to learn key skills, and they would not get it if their parents were relatively well-off.
People would also have to work longer to qualify for a boosted rate of Jobseeker's Allowance, as part of moves to restore the contributory principle to the system.
He rejected Conservative claims that the changes would cost money, insisting Labour was offering "big changes, not big spending". Aides said the reforms would save the government £65 million a year.
The reforms mirror proposals in a Condition of Britain report released by the IPPR think-tank, but Mr Miliband rejected one of the report's key recommendations for a freeze on Child Benefit for over-fives.
The commitments, unveiled in a speech to the IPPR in east London, come with Mr Miliband facing persistent questions about his leadership.
His personal ratings have been lagging significantly behind those of David Cameron, while a poll for Prospect magazine found voters thought his brother David would make a better prime minister.
Last night former cabinet minister Lord Mandelson appeared to offer only lukewarm support, saying Mr Miliband was "the leader we have" and urging him to offer a more pro-business agenda.
Mr Miliband sought to shrug off the former business secretary's comments, saying: "I always welcome advice, whatever source it comes from."
But he insisted that his party could not act as "continuity Labour", picking up from where it left off in 2010, but must respond to the problems of inequality and in-work poverty which had not been solved by New Labour.
Asked about his poll ratings, Mr Miliband said: "I knew when I took this job on that we were going to have a tough fight, because we are trying to defy the historical odds, which are that governments who lose elections don't tend to be one-term oppositions. We are in a position to defy those odds."
He added: "In the end, I have a big cause that I am fighting for and it is a tough fight, but I am determined that we win this fight. I am determined we win this fight not for the Labour Party, I am determined we win it for the British people.
"I didn't take this job because I thought it would be a walk in the park, I fought for this job because I thought it was important and I thought I had something distinctive to say about how we can change this country, and I believe that more now than I did three and a half years ago.
"I relish the next 10 months, I relish the opportunity to fight for my vision for the country."
Mr Miliband said his planned reforms would be introduced at a time of scarcity, when "the next Labour government won't have money to spend".
"The situation means we can't just hope to make do and mend," said the Labour leader. " We can't just borrow and spend money to paper over the cracks.
"The old way of doing things won't work any more. Instead, we need big, far-reaching reform. Which means big changes, not big spending.
"Reform that can reshape our economy, so that hard work is rewarded again. Rebuild our society, so that the next generation does better than the last. And change our country so that the British people feel it is run according to their values."
Labour would increase the rate of contributory Jobseeker's Allowance (JSA) for people who had spent significant time in work before becoming unemployed, he said.
But to fund a mooted rise from £71 to £100 a week, the period of employment needed to qualify would be extended - potentially from two to five years.
"To properly reward hard work and effort, we need contribution to be at the heart of our welfare system," said Mr Miliband.
"We talk about the problem of people getting something for nothing. And we are right to do so. But there is a problem that politicians rarely talk about of people getting nothing for something.
"How many times have I heard people say 'For years and years I paid in and then, when the time came and I needed help, I got nothing out'?
"Rewarding contribution was a key principle of the Beveridge Report. And it is a key intuition of the British people... W e should not allow the contributory principle to recede still further. Instead, we should strengthen it."
David Cameron has already suggested barring under-25s from receiving any benefits unless they are "earning or learning" - but Labour has dismissed the pledge as too blunt an instrument and lacking detail.
Instead, Mr Miliband backed IPPR proposals to scrap out-of-work benefits for those aged 18-21 and replace them with a "youth allowance" of the same value - currently around £57 a week.
They will not get the money if their skills are considered inadequate and they refuse to sign up for training, or if their parents' income is too high. People with small children or disabilities will be exempted from the rules.
"A Labour government will get young people to sign up for training, not sign on for benefits," said Mr Miliband.
There was speculation that Labour would endorse a similar scheme from the IPPR before Christmas, which would have made benefits conditional up to the age of 25. But party sources said its Jobs Guarantee scheme made such a move unnecessary.
That policy would see 18 to 24-year-olds who have been unemployed for a year offered a taxpayer-funded job for six months, with those who refuse losing benefits.
The sources indicated that those receiving "youth allowance" but still looking for work will still be eligible for the Jobs Guarantee scheme after a year.
Shadow work and pensions secretary Rachel Reeves said Labour's position was a "very different policy" from the one proposed by the IPPR last year, which she had spoken out against.
"In October the policy was all 18 to 25-year-olds, regardless of whether they had worked previously, regardless of whether they already had qualifications - A-levels, vocational qualifications, university degrees - and it was also restricting housing benefit for everybody under the age of 25," she told the BBC Radio 4 Today programme.
"So the policy today is a very different policy."
Conservative Party chairman Grant Shapps said: "This is just a recipe for more spending on welfare, more borrowing - and more taxes to pay for it. That's exactly how Labour got us into a mess in the first place.
"Ed Miliband has no economic plan. All he offers is more of the same old Labour - and Britain would have a less secure future as a result."
Lord Mandelson told BBC2's Newsnight last night that Mr Miliband had brought forward eye-catching policies, but now had a year before the general election to wind them into a "convincing, vivid narrative".
"Having policies, though, without those being drawn together into a convincing vivid narrative, a story about yourself, who you are what you stand for and what you are going to do for people in the country, is really not enough," said the Labour peer.
He said the emphasis on social justice needed to be balanced with an explanation on how Labour would encourage growth and jobs,"which he has a year to do".
Asked whether Mr Miliband was the best leader the party could have, Lord Mandelson replied: "In my view he is the leader we have and therefore the leader I support, and somebody who I believe is capable of leading the party to victory."
He said Mr Miliband was "choosing a different course" from ex-prime minister Tony Blair, adding that it "may work".
Meanwhile, former home secretary Alan Johnson told the New Statesman magazine that the Labour leader was "not as able to connect (with people) as strongly" as his brother David.
"It's not his strong point," he said. "I can't pretend that, knocking on doors, people come out and they're really enthusiastic about Ed."
Ms Reeves insisted that Mr Miliband would be a "great prime minister", telling Today: "I backed Ed from when he stood for the leadership of the party... no, I didn't make a mistake.
"Absolutely, he is doing the right thing. In a year's time I think that Ed will be prime minister and I think he will be a great prime minister."
British Chambers of Commerce executive director Adam Marshall said: "Giving 18 to 21-year-olds training allowances rather than unemployment benefits will be an attractive idea to many in the business community, who express concern that young people often don't have the right skills to succeed in the world of work.
"The acid test will be whether these proposals help to address the confidence gap between businesses and young people. As our own Skills Manifesto has shown, companies aren't confident when recruiting school-leavers and young people feel that businesses don't give them a chance.
"If a new training-focused youth allowance can help tackle the confidence gap between young people and firms, it could receive real support from companies across the UK."
Laurence Maples, of Youth Fight for Jobs, said: " Just like the Tories, Labour is all too keen to place the blame for unemployment at the feet of those victim to it. Young people are not suffering from a lack of skill, talent or determination, we are suffering from a lack of available jobs.
"Labour pathetically attempting to out-Tory the Tories is the last thing we need. If Miliband wants young people's votes he'll have to do better than this. A commitment to a mass programme of job creation, to create socially useful jobs paid a living wage, would be a better place to start."
Writing on his blog, Gordon Brown's former spin chief Damian McBride warned that there were no "fighters" in Mr Miliband's top team.
"There are many positive things to say about the people managing Ed Miliband's operation and running Labour's campaign. They are well-spoken, well-read, well-connected, and, if you stay on their right side, quite genial. You'd feel safe sitting them next to your mum at a wedding," Mr McBride wrote.
"But what they are not is fighters. They will never give their press team and foot-soldiers the ammunition required to win the next 40 weeks in the media and on the doorstep, not just because they lack an understanding of what might do the trick, but also an appreciation of why doing so matters.
"Not when they could be attending a Thomas Piketty symposium instead."
He said Mr Miliband, like Mr Cameron, was "guilty of recruiting his innermost circle of advisers entirely in his own image".
"That's all right in peacetime if it helps him shape his political philosophy and refine his personal blueprint for government. It might even be OK if basking in a large majority in office," Mr McBride went on.
"But with an election to win - an election Labour can win - Ed urgently needs to add some 'wartime consiglieres' to the mix, not in place of the very capable and trusted people he's got, but working alongside them."