Andy Burnham has asked Health Secretary, Jeremy Hunt, to substantiate his claims that the Labour MP hid hospital failings in a "cover up".
He said he was concerned that disclosure rules had been broken and challenged Mr Hunt to apologise.
It is impossible to see how you can claim this mounted to a cover-up," he told him in a letter.
I therefore ask you, by the end of today, to provide me with evidence to substantiate your assertion.
If you fail to provide such evidence, I will require a full retraction and public apology. If that is not forthcoming I will consider further action.
More top news
The law should be changed to allow assisted dying, the country's former top lawyer has said.
The victims of the Shoreham air crash will be remembered in a series of events today to mark one week since at least 11 people were killed.
Jose Mourinho's treatment of Chelsea team doctor Eva Carneiro is to be discussed by a FIFA committee, it has emerged.