Rebekah Brooks, the former chief executive of News International, has been released on police bail following her arrest over allegations of corruption and phone hacking.
The 43-year-old went to a London police station by appointment at midday on Sunday and was released at about midnight. She has been bailed until late October.
Mrs Brooks was held as part of Operation Weeting, launched to investigate claims that newspaper journalists hacked into phones, and over allegations of illegal payments to police officers.
The arrest came after she resigned as chief executive of News International (NI) on Friday, which she said was to avoid distracting attention from the company's efforts to "fix the problems of the past".
Mrs Brooks was editor of the now-defunct News Of The World (NOTW), which was published by NI, when missing schoolgirl Milly Dowler's phone was allegedly hacked and messages deleted. The teenager was later found murdered.
She is the tenth person to be arrested in connection with the NOTW scandal since detectives reopened their investigation earlier this year.
A separate investigation, Operation Elveden, is looking at allegations of inappropriate payments to police.
Mrs Brooks' spokesman earlier said she was contacted by police with regards to arranging an appointment some time after she resigned.
"Rebekah has been offering to help police with their enquiries since January. The police explicitly said they did not need to speak to her.
"As late as last week the police still maintained they did not need to speak to her.
"However following her resignation and the announcement that she would attend the Select Committee meeting on Tuesday the police changed their course of action and told Rebekah they did want to speak to her.
"The arrest puts her in a difficult position in terms of her scheduled appearance at the meeting on Tuesday. Rebekah's lawyers will be seeking guidance as to the course of action in regards to attending the meeting on Tuesday."
Mrs Brooks is due to appear alongside News Corporation chairman Rupert Murdoch and his son James, who is chairman of NI, before the Culture, Media and Sport committee on Tuesday.
But Mark White, home affairs correspondent, said the arrest throws into doubt the highly anticipated appearance.
He said: "When there are some difficult questions coming her way, that gives her the opportunity perhaps not to be as candid with those questions as the committee might like her to be."
Committee chair John Whittingdale has said he does not know if Mrs Brooks will still appear before MPs.
Mark Lewis, the lawyer for the Dowler family, told Sky News he thinks the timing of the arrest "stinks".
Mr Lewis said: "To be arrested two days before a committee at which she was possibly going to be at one time the only person from News Corp giving evidence looks deliberate."
Media commentator Steve Hewlett told Sky News: "The fact that she's been arrested is not entirely surprising - I don't think it should be taken to mean there is necessarily evidence pointing to her direct knowledge of the events.
"But I imagine it is a matter of procedure really - that the inquiry is getting serious."
Ex-NOTW political editor David Wooding has tweeted that "senior execs at NI had no idea Rebekah was about to be arrested when she quit last week".