The Labour leader told a party press conference: "I fear for the future of the NHS if the Tories get back in. Brick by brick they have been dismantling the foundations on which the NHS stands. If they win, they will have a licence to kill the NHS as we know it. They must be stopped."
Mr Blair based his attack on leaked material from a joint private medical insurance deal, under discussion between Norwich Union and the NHS trusts. He said it supported his charge that the Tories were planning "a major expansion in the use of private health insurance as part of the two-tier NHS".
The document speaks of targeting patients who feel guilty about abandoning the NHS but who would be prepared to pay to jump the queue for treatment. It says the new private-care policy would be sold to "socially responsible dinkies [double income, no kids] and empty nesters, such as teachers, public service personnel, politically right-on professionals and your Matthew Hardings" - a reference to the millionaire chairman of Chelsea Football Club who gave large sums to the Labour Party before he was killed in a helicopter accident last year.
The document says these people are seeking "an acceptable way to go private" who want speed of treatment but are "far less concerned about the added frills." Their objections could be overcome by convincing them that they are supporting their local hospital and making a contribution to NHS patients, it says.
Mr Blair said in response: "Unless the people of Britain use their vote on Thursday to send a Mayday warning, there will not be a National Health Service worth the name in five years' time."
Chris Smith, Labour's health spokesman, later described the arrangement as a "sweetheart deal", prompting Stephen Dorrell, the Secretary of State for Health, to issue a denial through Tory party headquarters. Mr Dorrell added that he had been in touch with the chief executive of the NHS Confederation, which represents NHS trusts and health authorities, who had told him: "I have informed the co-chairs of the NHS Confederation that, in my view, it would not be appropriate for the confederation to be involved in promoting private medical insurance."
The leaked Norwich Union document, dated 9 December last year, says the price of policies that might be offered should be "artificially lowered to attract new business" in the first year. But it warns of a "potential backlash by the NHS Trust Federation should we need to push through high price increases in years two and three".
The NHS Trust Federation opened negotiations with several insurance companies last year to seek ways of defending its pay-bed income of over pounds 200m a year.
Some private insurers such as Bupa have adopted aggressive marketing tactics excluding NHS hospitals from their schemes and offering bonuses to consultants who treat private patients in non-NHS hospitals.
The federation merged last month with the National Association of Health Authorities and Trusts to form the NHS Confederation. Yesterday, the confederation said any scheme involving a private provider would have to be "re-thought" after the election.