David Cameron will today invite the public to vote against him at the next general election if he fails to keep his promises to protect the National Health Service.

The Prime Minister will put his reputation on the line as he tries to allay fears that the Government's reforms could lead to the back-door privatisation of the NHS.

In a "trust me" speech in London, he will argue that no change is not an option and would threaten "the precious principle of free healthcare for all who need it, when they need it". He will say: "We have to change the NHS to avoid a crisis tomorrow too. This is what will happen if we don't. More overstretch, more over-crowding, the NHS buckling under the pressure of an ageing population and the rising cost of treatments."

Mr Cameron is trying to bridge a divide between the Deputy Prime Minister, Nick Clegg, who is demanding changes to the reforms, and the Health Secretary, Andrew Lansley, who is trying to prevent his plans being filleted. Defending the Government's "pause" on the NHS and Social Care Bill, Mr Cameron will say it is worth delaying to get it right.

He will offer five personal guarantees: that the Government will not endanger universal coverage; will not break up or hinder efficient and integrated care; will keep waiting times low; will increase spending on the NHS; and will not "sell-off" the NHS to create a US-style private system, but will ensure competition benefits patients.

The Prime Minister will describe the pledges as "guarantees you can hold me to and that I will be personally accountable for". His move echoes the "reassurance strategy" he adopted after becoming Tory leader in 2005, when he highlighted his own family's use of the NHS.

Insisting that too much money is being wasted on "empty bureaucracy" rather than frontline services, he will say: "In the past two decades, NHS spending has more than doubled in real terms from £38bn to £103bn. That injection of money has been right – but can we really say that the improvement in service has reflected that increase? Can we really say we're getting value for every pound that we spend?

"We're seeing a deep divide between health and social care that is causing serious problems for vulnerable, often elderly, people and their families. We're hearing too many stories about patients being moved from pillar to post, getting lost in a labyrinth of letters and appointments and referrals, when what they really want is to be in the driving seat." He will describe as "damning" last week's reports that the "standard of care in some of hospitals was appalling, with elderly patients left unfed and unwashed".