Philip Rutnam quit in February after a series of clashes with the home secretary.
Home secretary makes call for unity after accusations from the most senior official at the Home Office.
The prime minister says he "absolutely" has full confidence in Patel despite the explosive resignation of top Home Office mandarin Sir Philip Rutnam.
HuffPost UK revealed on Thursday that tensions had risen between Sir Philip Rutnam and Priti Patel after he refused to sack a colleague on Christmas Eve.
Permanent secretary Sir Philip Rutnam's refusal to carry out home secretary's order to move Home Office communications chief deepened rift between the pair.