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
A couple have taken a step closer to bringing a child born in India via surrogacy to their home in the UK.
The Yellow Cloud electric guitar that Prince played in numerous concerts has sold for more than £100,000 at auction.
An athlete had his eye pierced with a javelin after running and tripping onto the spear during a competition.