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
Wednesday unsavoury incident involving Jonny Evans and Papiss Cisse is the latest in a long line of gob-related football controversy
Middle managers dubbed 'the permafrost' have been blamed for keeping female intelligent agents down. They have been urged to change.
Holidaymakers are set to be banned from drinking on the streets of Magaluf this summer.