Former Sun and News of the World editor Rebekah Brooks is expected to lift the lid on her close relationship with the Prime Minister in evidence to the Leveson Inquiry today.
David Cameron is said to have texted Brooks, telling her to "keep her head up" after she resigned from News International last July.
It has also been claimed that the 43-year-old former editor sent Cameron more than 12 text messages a day.
After her editorships Brooks went on to become chief executive of Rupert Murdoch's UK newspapers division News International in September 2009 until she resigned in the wake of the hacking scandal last July.
She and racehorse trainer husband Charlie are key members of the influential Chipping Norton set, which also includes Cameron and his wife Samantha, Top Gear presenter Jeremy Clarkson, and Murdoch's daughter Elisabeth and her PR guru husband Matthew Freud.
The inquiry has already heard that Brooks regularly met Cameron and other top politicians along with Rupert and James Murdoch.
She hosted a Christmas dinner on December 23 2010, just two days after Business Secretary Vince Cable was stripped of his responsibility for media takeovers for saying he had "declared war" on the Murdochs' News Corporation empire.
Brooks's wedding on 13 June 2009 was attended by Cameron and former prime minister Gordon Brown, and in March Cameron was forced to admit that he rode a retired police horse loaned to Brooks by Scotland Yard from 2008 to 2010.
An updated biography of Cameron: Practically A Conservative, claims he told Brooks she would get through her difficulties just days before she stood down over the phone hacking scandal.
There has speculation that the Leveson Inquiry could release emails and text messages sent between Cameron and the former News International chief executive.
According to Daily Telegraph columnist Peter Oborne, Brooks has kept all the texts she received from the Prime Minister.
Brooks has twice been arrested by Scotland Yard detectives investigating allegations of phone hacking, corrupt payments to public officials, and an attempt to pervert the course of justice. She was bailed and has not been charged.
She will not be questioned about anything that could prejudice the continuing police investigation into phone hacking or any potential future trials.
Brooks' appearance could raise awkward questions for the Prime Minister as he tries to relaunch the coalition in the wake of bruising local election results last week.
Yesterday his former News of the World editor Andy Coulson was asked about how he came to be the PM's spin doctor.
Cameron said last July that "with 20:20 hindsight" he would not have hired Coulson in May 2007, four months after he resigned from the Sunday tabloid over the jailing of royal reporter Clive Goodman for phone hacking.
Coulson, 44, became Downing Street's communications chief in May 2010 but quit eight months later, saying controversy over the hacking scandal was making his job impossible.
Speaking publicly for the first time since being arrested by Scotland Yard on suspicion of phone hacking and corruption last July, he told the inquiry there was no "grand conspiracy" between the Tories and the Murdoch empire.
Coulson, who has been bailed and not charged, also admitted he had failed to declare a £40,000 shareholding in News Corporation while he was in Downing Street.
Cameron set up the Leveson Inquiry last July in response to revelations that the now-defunct News of the World hacked murdered schoolgirl Milly Dowler's phone after she disappeared in 2002.
The first part of the inquiry, sitting at the Royal Courts of Justice in London, is looking at the culture, practices and ethics of the Press in general and is due to produce a report by October.