The former editor of both the News Of The World and The Sun, Rebekah Brooks, will give evidence to the Leveson Inquiry later.
Mrs Brooks resigned as chief executive of the papers' parent company News International in the wake of the phone-hacking scandal, though she maintains she was unaware it was going on.
Mrs Brooks and her husband, race horse trainer Charlie Brooks, are friends and country neighbours of the Prime Minister - and Lord Justice Leveson is likely to want to learn whether that friendship ever strayed into professional back scratching.
Mrs Brooks' former boss, James Murdoch, has already told the inquiry that he discussed the controversial planned takeover of BSkyB by News Corp with the Prime Minister at a Christmas dinner at the Brooks' family home in the Cotswolds.
And David Cameron was forced to admit earlier this year that he had ridden a retired police horse being looked after by Rebekah Brooks at the home near Chipping Norton.
Mr Cameron is the area's local MP and his constituency home is less than four miles away from the Brooks'.
He has known Mr Brooks for many years and the inquiry lawyer will ask questions about the propriety of such a close relationship between an influential newspaper executive and the man who runs the country.
A new book has claimed that Mrs Brooks and the Prime Minister used to text each other regularly.
And her former deputy - and Mr Cameron's ex-communications director - Andy Coulson has already given evidence about the closeness of the pair.
Mr Cameron even attended Mrs Brooks' wedding in 2009 before he was Prime Minister - though the man who was in residence at Number 10, Gordon Brown, also attended the nuptials.
It too gives rise to questions about the professional and personal relationships between senior politicians of all parties and media executives.
Mr Cameron has been Prime Minister for exactly two years. How he starts the third may be shaped in some small part by what his good friend Mrs Brooks tells the inquiry, that the PM himself set up, about their relationship.