The Tories can fund their pledge to dramatically boost NHS spending because of "long-term decisions" taken in the past five years, David Cameron insisted today.

Speaking on a visit to the Witney Community Hospital, the Prime Minister said the policy was possible because of the "strong economy" built by the coalition.

The Tories have faced claims they have made a huge unfunded promise to fully meet the calls of NHS chief executive Simon Stevens, but the party is pointing to the more than £7 billion a year increase in spending in the last Parliament.

The Conservative manifesto next week will include a promise to ensure that by 2020, NHS spending is £8 billion a year higher in real terms.

Mr Cameron said: "I want an NHS that continues to expand and improve and provide great care, that continues to save lives.

"It has always been there for me and my family and I want it there for everyone's families.

"That's why we are making this decision today, to fund this plan in full, and we are able to do that because we have a strong economy and because we have taken the long-term decisions necessary to put the NHS first.

He added: "For us the NHS has always been a priority because we want the NHS to expand, to improve, to provide the great life-saving treatments, to have that great care.

"That's what we have done in the last Parliament, that's what we are going to do in this Parliament, and we are able to fund the NHS's own plan in full because we have a strong economy.

"A strong economy that is linked to the difficult long-term decisions we have taken elsewhere."

Labour and the Liberal Democrats led criticism of the Tory pledge, insisting Mr Cameron's party had not spelt out how it would find the money.

Labour accused the Tories of making "fantasy promises" that were unfunded and could not be kept, with shadow chief secretary to the Treasury Chris Leslie saying no-one would believe a word.

Liberal Democrat Health minister Norman Lamb warned the Conservatives' "ideological obsession" with the size of the state meant they could not afford the promise, adding he felt the Tories were trying to "pull the wool over the British public's eyes".

Tory sources said the Labour criticism was like "groundhog day", pointing to Labour's forecasts at the start of the last Parliament that it would be impossible to cut the deficit, cut taxes and increase spending on the NHS at the same time.

Mr Cameron has said the commitment means that over-75s will be guaranteed same-day access to GPs, patients will be able to see doctors out of regular office hours and the NHS will provide a full range of services seven days a week.

The move, on a key election battleground, comes after a week of bitter skirmishing that saw the Tories question Ed Miliband's character - reminding voters that he "stabbed his brother in the back" to win the Labour leadership.

As part of the plan drawn up by Mr Stevens, £22 billion of efficiency savings will also need to be found each year. This is a mixture of cutting waste, efficiencies and preventative medicine.

On his visit, Mr Cameron met nurses at a nurses' station and the hospital's EMU - the Emergency Multidisciplinary Unit.

He was told patients were treated in the unit for as little as a few hours, with the goal of reducing demands on accident and emergency units.

Mr Cameron told the staff the Witney unit was a model which could be followed across the country as the NHS moves to a more flexible, seven-day service.

Defending the announcement and outlining the plans, Health Secretary Jeremy Hunt told the BBC Radio 4 Today programme: "Confidence is the issue because on May 7 voters will have a choice.

"If you don't want to take the commitment David Cameron and George Osborne are making, look at their track record."

He said the NHS had "weathered the storm" of the "most difficult financial crisis since the Second World War".

He went on: "Look at the evidence as to what has happened to the British economy. In the end a strong NHS needs a strong economy.

"If you are going to tear up the economic plan, then that is the biggest risk for funding to the NHS."

The blueprint unveiled by Mr Stevens last October predicted that if health spending rose only in line with inflation, growing demand for care would leave NHS England with a £30 billion funding gap by 2020.

The chief executive said around £22 billion of that could be met through efficiencies - but the rest would have to come from government coffers.

The Liberal Democrats were the first to commit to funding the plan and insist they are the only party which has clearly said how it would be paid for.

Speaking to the Today programme, shadow health minister Liz Kendall criticised the Tories' "fantasy promises" and claimed they were "panicking".

She was asked several times whether her own party would match the £8 billion figure, but would not commit to it.

She said: "I'm certainly not going to make a promise that is fantasy funding," but she went on to add her party would do "whatever it takes".

Jennifer Dixon, chief executive of the Health Foundation thinktank, said: "Today's pledge of a minimum of £8 billion real-terms increase by 2020/21 is very welcome as it gives some certainty to the NHS in England at a time of significant challenge.

"However, this is the absolute minimum the NHS will need. The £8 billion figure assumes the health service can improve its productivity by 2% to 3% each year to 2020/21, which is very optimistic.

"New research by the Health Foundation found that NHS hospitals have only improved efficiency at an average rate of 0.4% a year over this parliament. This is why the NHS urgently needs a transformation fund to help test and invest in new and more efficient models of care."

Nuffield Trust chief executive Nigel Edwards said Mr Osborne's pledge was a welcome "vote of confidence" in NHS England's five-year transformation plan, which envisages efficiency savings of £22 billion.

But Mr Edwards warned: "Running to stand still is all that an additional £8 billion - coupled with £22 billion of efficiency savings - will allow. These figures are based on keeping the amount of NHS funding flat for each person in the population, taking into account their age and healthcare needs.

"This means that, even if the NHS can climb the mountain ahead and achieve unprecedented levels of efficiency savings, finding the money needed for a number of very expensive new drugs and treatments in the pipeline will be extremely difficult."

Ukip leader Nigel Farage described the situation as a "game of poker" in which whatever one party says, another party raises it.

He told Sky News: "It's all well and good to talk about an extra £8 billion by 2020, but there has been absolutely no suggestion provided as to where the money is going to come from.

"The real problem the NHS has got is that our population is rising so rapidly that we find ourselves desperately short of enough GPs and A&E facilities.

"If we were to keep our population at a reasonable number, I suggest it might be easier to keep the NHS free at the point of delivery."

Mr Farage said that principle was one the British people believed in, adding: "They really do. I don't see that changing for the foreseeable future.

"The problem is how are they going to fund it. I think what has been promised overnight by the Conservative Party, it's as if there's a money tree."