A Scotland Yard chief forced to resign over the phone-hacking scandal gives evidence today at the Leveson Inquiry into press standards.
John Yates is due to be questioned alongside Andy Hayman, the officer in charge of the original hacking investigation in 2006.
Yates and Mr Hayman, both former assistant commissioners, were involved in heated exchanges with MPs at the peak of the crisis last summer.
Hayman, who became a columnist for News International paper the Times after retiring from Scotland Yard, was called a "dodgy geezer" by an MP on the Commons Home Affairs Select Committee.
Yates, meanwhile, quit the Metropolitan Police in July after coming under criticism for links to a News of the World (NotW) executive.
Former deputy assistant commissioner Peter Clarke will also face questions after reports he gave Rupert Murdoch a face-to-face briefing over an ongoing anti-terror operation.
Clarke was understood to have met the media tycoon with Rebekah Brooks and Andy Coulson, then editors of The Sun and the News of the World, in 2004.
Both journalists have since been arrested and bailed on suspicion of phone-hacking.
Scotland Yard's original phone hacking inquiry resulted in the jailing of NotW royal editor Clive Goodman and private investigator Glenn Mulcaire in January 2007 after they admitted intercepting voicemail messages left on royal aides' phones.
However the Met was heavily criticised for limiting the scope of the investigation despite evidence from Mulcaire's notebooks that there could be thousands of hacking victims.
Yates came under fire when he decided not to reopen the phone hacking inquiry after the Guardian published a story in 2009 revealing the illegal practice was far more widespread than previously believed.
He will give evidence via videolink from Bahrain, where he is overseeing reform of the police force.
Prime Minister David Cameron set up the Leveson Inquiry last July in response to revelations that the NotW 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 September.
The second part, examining the extent of unlawful activities by journalists, will not begin until detectives have completed their investigation into alleged phone hacking and corrupt payments to police, and any prosecutions have been concluded.
Former Scotland Yard commissioner Sir Paul Stephenson, who also resigned over the phone-hacking scandal, was due to appear before the inquiry today, but his session has been postponed until Monday.