Ed Miliband has launched Labour's campaign for the May 7 general election with the promise of a "double lock" to safeguard the future of the NHS.
A Labour government would act to ensure health services are no longer "threatened" by privatisation and to provide £2.5 billion of extra investment, funded by taxes on expensive properties and tobacco companies and a crackdown on tax avoidance, he said.
Under Labour's plans, a new profit cap - usually 5% - would be set on outsourced healthcare contracts worth over £500,000, private companies would be blocked from "cherry-picking" lucrative treatments and the NHS would become the "preferred provider" for all services.
Speaking at the Olympic Park in east London, Mr Miliband said: "We need to rescue our NHS from this Government, and we will.
"Just think about how far backwards the NHS has gone in the last five years. People waiting longer and longer to see a GP. Ambulances queuing up outside hospitals, because A&E is full. Even a treatment tent erected in a hospital car park.
"For all the promises, for all the air-brushed posters, David Cameron has broken his solemn vow to the British people when it comes to our NHS.
"And that is even before their plan for the next few years. Cuts even deeper than those we have seen in this last five.
"Well, that's not the future I believe in. That's not the future you believe in. And it's not the future the British people can believe in either.
"So we will turn around our NHS. With a Labour government there will be a new double-lock to protect our National Health Service, guaranteeing proper funding and stopping its privatisation."
Mr Miliband repeated his promise that Labour would repeal the "market framework for the NHS" established in the Coalition's Health and Social Care Act, which he said had forced competitive tendering of services, with a third of all contracts going to the private sector since its implementation.
And he said he would use money from the "mansion tax" on properties worth more than £2 million, a new tobacco company levy and the closure of hedge fund tax loopholes to hire 20,000 more nurses, 8,000 more doctors, 5,000 new care workers and 3,000 extra midwives.
The Labour leader said yesterday's TV interviews had revealed David Cameron as "a rattled Prime Minister, running from his record" and "living in a different world".
He said the Conservative leader had suggested that "soaring" use of food banks was due to more effective advertising by Government, and that zero-hours contracts had increased because workers want them.
"Then he says 'Oh no, I couldn't live on a zero-hours contract'," said Mr Miliband.
"I say to the Prime Minister, if it's not good enough for you, it's not good enough for the people of Britain."
Mr Miliband said the choice for voters on May 7 was between pessimistic Tories and a Labour Party which had "a spirit of optimism" at the heart of its appeal.
"The Tories say: this is as good as it gets," said the Labour leader. " We say: Britain can do better than this."
The fundamental choice at the 2015 election was between "a Tory government that looks out only for the few, o r a Labour government that will stand up for working families in every part of our country", he said.
Speaking in the viewing gallery of the distinctive red ArcelorMittal Orbit landmark at the Olympic Park, Mr Miliband admitted that the election race was going to be neck-and-neck and could "come down to the wire".
He said: "I know our opponents will throw everything they have our way, because they're desperate to hang on to power.
"But we know we can win this fight on behalf of the British people.
"We know we must stand up for working families. We know we must change Britain."
Conservative Health Secretary Jeremy Hunt said: "If you bankrupt the economy like Labour did last time, then you'll put our NHS at risk. We can only have a strong NHS if we have a strong economy, but Ed Miliband doesn't have an economic plan.
"We all know Labour want to 'weaponise' the NHS but this is another policy from Ed Miliband that looks ill-thought through. It risks higher infection rates, higher waiting times and chaos for our NHS. This incompetence is exactly why Ed Miliband is simply not up to the job.
"Independent commentators confirm that use of the private sector has grown at half the rate under this Government as it did under Labour, so this is no more than a gimmick to scare people about privatisation that isn't happening.
"The real issue for the future of the NHS is how to fund the growing needs of an ageing population, and by tearing up an economic plan that is working, Labour would threaten the real increases in funding that Conservatives are promising. No-one can deliver a strong NHS without a strong economy."
In response to questions from reporters, Mr Miliband said of the profit cap: "We do think it's right to say, when private sector companies are involved, we don't get excess profits which is draining money away from the National Health Service.
"Look, I think this is going to be common-sense thing that people will think is the right thing to do. I hope other parties adopt this because it's the right thing to do for the future of our NHS."
On people paying for private healthcare, he added: "What is the real lesson of the past on this - the lesson before 1997 and a lesson I fear for the future if David Cameron were to get back in?
"It's that people end up having the option of either waiting for an unacceptably long time or having to go private - that that becomes the offer on the table for people.
"So I think there is a real fear that that becomes the option for people and you don't have a health service that is properly serving people so there's a big choice at this election and a big choice about the health service on the ballot paper."
Shadow health secretary Andy Burnham said the election needed to be Mr Cameron's "day of reckoning" on the health service, as he insisted nobody in the country had given the PM "their permission to put the NHS up for sale".
Mr Burnham did not provide a number or estimate when asked how many companies would fall foul of Labour's new rule.
Asked to name his target for private sector involvement in the NHS , Mr Burnham told BBC Radio 4's World at One: "The market should be removed from the National Health Service.
"We should set the public NHS as the preferred provider of services so I want to see a significant reduction.
"I've said to you in this interview that there is a role for other providers to support the NHS - providers in the voluntary sector and in the private sector. So there is a role.
"But I see that role as a supporting role, not a replacement role.
"I'm not going to set arbitrary percentages - it's a decision for people at local level to decide whether they need to bring in additional capacity or innovation to support the NHS.
"But I am quite clear that we need to see that percentage fall because if we allow the market to carry on advancing into the NHS it will in the end destroy everything that's precious about it. It will unpick the fabric of a National Health Service."
Mr Burnham added it was "time to call time on the market experiment" in the NHS.
Royal College of Midwives communications director Jon Skewes said: "We welcome the vital extra money for the NHS that Labour is pledging to provide.
"The RCM is very pleased to see the clear and specific commitment to 3,000 more NHS midwives. So long as the number of births does not start to rise again, these extra midwives could potentially eliminate England's long-standing midwifery shortage. That would be a big achievement, and we hope that all parties will match this pledge."
British Medical Association council chair Mark Porter said: "The test of any health policy should be whether it benefits patients, yet 95% of doctors do not believe the quality of patient care has improved under the Health and Social Care Act 2012.
"Proposals to remove the most damaging elements of the Health and Social Care Act 2012, to support more joined-up care and to prevent the private sector from cherry-picking the most profitable services are a step in the right direction. The BMA wants a publicly-provided and funded health service, and believes the NHS should always be the preferred provider."
Liberal Democrat health minister Norman Lamb said: "Liberal Democrats in government put an end to Labour's sweetheart deals with the private sector, while the independent King's Fund has poured cold water on Labour's claims about privatisation.
"Only the Liberal Democrats have a credible plan to invest the £8 billion per year by 2020 that NHS bosses say is needed.
"A private sector profit cap sounds appealing but the devil would be in the detail and a simplistic policy could cause chaos for local hospitals."