Health Secretary Alan Johnson has been focused "on another job" during his time in office, shadow health secretary Andrew Lansley is to say.
He is the "postman that hasn't delivered" on the NHS and has been more interested "in his own ambition", Mr Lansley will claim in a speech.
Mr Johnson has been tipped as a strong favourite to succeed Prime Minister Gordon Brown as Labour leader.
In his speech to the Royal Society for the encouragement of Arts, Manufactures and Commerce (RSA), Mr Lansley will say: "He didn't want to be Health Secretary in the first place. He has been planning to move on to another job in politics for most of the time he's been there.
"He seems more focused on promoting his own ambition than promoting an agenda that will help patients.
"When you look at his record there has been a complete failure to deliver on crucial reforms started by Tony Blair, and a failure to deliver on major threats to our nation's health - whether that's obesity, the health of the poor, or the dignity of patients being forced into mixed sex wards.
"No delivery for patients and no delivery for health professionals - he is the postman that hasn't delivered."
Mr Lansley will argue that, in his view, there is no greater responsibility than being entrusted with improving the NHS.
"People could be forgiven for thinking that the present incumbent doesn't share this view. Instead of driving through key Blairite reforms in the NHS - ones which we have long supported of patient choice and of competition - Alan Johnson's eye has been focused on another job. And the NHS is suffering as a result."
Mr Lansley will say reforms begun under Mr Blair have "completely stalled" under Mr Johnson and Mr Brown.
