David Cameron has admitted the general election is on a "knife edge" as he used his first big speech of the campaign to launch a blistering attack on Ed Miliband.

Addressing activists at the Conservative spring conference in Manchester, the Prime Minister sought to cast the contest as a personal battle between himself and a Labour leader he said was not up to the job.

At the same time, he sought make a positive pitch to voters setting out plans for a "truly seven-day-a-week" NHS in England if the Tories regain power after May 7.

Mr Cameron that in an election race which "can only be cut two ways" - with either him or Mr Miliband entering No 10 - it was right that he should focus on his opponent, saying "the personal is national".

He accused the Labour leader of planning to "crawl up Downing Street on the coat-tails of the SNP" and warned that his spending plans would wreck the economic recovery.

"Now five years in this job teaches you some things. I know what this role needs - and frankly, I don't think Ed Miliband has it," he said.

"Some might say 'Don't make this personal', but when it comes to who's prime minister, the personal is national.

"The guy who forgot to mention the deficit could be the one in charge of our whole economy. The man who is too weak to stand up to the trade unions at home could be the one facing down our enemies abroad.

"The leader who thinks leadership is climbing aboard the latest bandwagon - he could be the one taking the make-or-break calls in the middle of the night."

He said that under Mr Miliband, Labour had become a "bunch of hypocritical, holier-than-thou, hopeless, sneering socialists" having betrayed its traditional values and its belief in the dignity of work.

"The truth is that Miliband's Labour Party isn't about liberating working people - it's about telling you what to do. The same old condescending, bossy, interfering, we-know-best attitude of the Hampstead socialist down the ages," he said.

"This isn't the party of working people - it's the same old party of hypocrisy, the party of two faces, the party of two Jags and now, yes, the party of two kitchens."

The Prime Minister was briefly heckled by a man in audience who later told reporters that he was a Conservative Party member but had voted Ukip at the last election because of Mr Cameron's betrayal on Europe and immigration.

On the NHS, Mr Cameron that a Conservative government would ensure patients across England would be given full access to hospital services seven days a week by the end of the parliament in 2020.

"For years it's been too hard to access the NHS out of hours. But illness doesn't respect working hours. Heart attacks, major accidents, babies - these things don't just come from nine to five," he said.

"And the truth is that you are actually more likely to die if you turn up at the hospital at the weekend. Some of the resources are not up and running. The key decision-makers aren't always there.

"With a future Conservative government, we would have a truly seven-day NHS.

But the plan came under attack from doctors' leaders who accuse the Prime Minister of "shameless political game playing".

Dr Mark Porter, who chairs the British Medical Association council, said the Conservatives had not even committed the funds needed to maintain existing services.

"With existing services stretched to breaking point, a majority of hospitals facing crippling budget deficits and frontline staff under extreme pressure, the NHS needs far more than just words to deliver extra care," he said.

"Without a detailed, fully-costed plan to provide the staff and resources needed to deliver more seven-day services, this is at best an empty pledge and at worst shameless political game playing with the NHS ahead of the election."

Labour's campaign vice chair Lucy Powell accused the Conservatives of misleading voters, saying their spending plans would mean "extreme" cuts to the NHS.

"On the NHS, David Cameron misled people in 2010 and he's misleading them again today," she said.

"When he pledges seven-day-a-week care in the NHS, people will remember that he did exactly the same before the last election, only to break his word. After five years of the Tories it's harder to see your GP and A&Es are in crisis."

A Liberal Democrat spokesman said that NHS England already had plans in place to move towards hospitals and GPs being open seven days a week.

"To keep this sustainable we need to back the NHS with more investment. Only the Liberal Democrats have a credible plan to invest the £8 billion per year by 2020 that NHS bosses say is needed," the spokesman said.

Ukip health spokesman Louise Bours said: "If the Tories were serious about the NHS they would have brought these changes in over the last five years they have been in government, but instead they have degenerated the NHS beyond all recognition."