Former News of the World editors Rebekah Brooks and Andy Coulson were having an affair for much of the time they were allegedly involved in phone hacking at the Rupert Murdoch-owned tabloid, a London court heard.
Prosecutor Andrew Edis said Thursday the affair, from 1998 to 2004, proved "they trusted each other" and supported charges that they conspired to hack phones to glean stories for the newspaper, which was renowned for its celebrity scoops until Murdoch closed it in disgrace two years ago.
Brooks edited the News of the World from 2000 to 2003, when her deputy Coulson took over as editor. A close confidante of Murdoch's, Brooks went on to become chief executive of his British newspaper operations while Coulson became Prime Minister David Cameron's media chief.
Both Brooks and Coulson deny hacking and related allegations in the high-profile trial at the Old Bailey, which also involves six other defendants including Brooks' current husband.
Her affair with Coulson was revealed in a love letter dated February 2004 that was found on her computer, the court heard. She apparently wrote it in response to Coulson's efforts to end the relationship.
Both were married at the time -- Brooks wed her first husband, actor Ross Kemp, in 2002. Coulson married in 2000 and has two children.
Edis said he made no "moral judgement" about the defendants, who sat next to each other in the dock, Brooks' head bowed and Coulson looking towards the prosecutor.
But the lawyer told the jury: "Mrs Brooks and Mr Coulson are charged with conspiracy, and when people are charged with conspiracy the first question a jury has to answer is, how well did they know each other? How much did they trust each other?
"And the fact that they were in this relationship -- which was a secret -- means that they trusted each other quite a lot with at least that secret, and that's why we are telling you about it."
In the letter, Brooks wrote: "The fact is you are my very best friend, I tell you everything, I confide in you, I seek your advice, I love you, care about you, worry about you, we laugh and cry together.
"In fact without our relationship in my life I am not sure I will cope."
The affair dominated newspapers on Friday, turning the tables on a couple whose former paper was renowned for exposing celebrity infidelities.
The Daily Telegraph, Guardian, Daily Mail and Independent all carried cover pictures of the couple, while the headline "Editors and lovers: couple at heart of hacking trial" topped the front-page of Murdoch title the Times.
The prosecution alleges that Brooks, Coulson and ex-managing editor Stuart Kuttner must have known about widespread hacking at the News of the World between 2000 and 2006.
Private investigator Glenn Mulcaire was paid £100,000 a year to work with an investigations team set up by Brooks, Edis said.
Mulcaire has since admitted phone hacking, as has another member of the team, senior journalist Greg Miskiw.
The prosecutor argued that Brooks, Coulson and Kuttner were trying to rein in the newspaper's budget and Mulcaire's contract would have stood out.
Mulcaire admitted ahead of the trial to the worst case of phone hacking, which targeted schoolgirl Milly Dowler after she disappeared in 2002. The 13-year-old was later found murdered.
Edis alleged that Coulson, Brooks and Kuttner "were criminally involved in the conspiracy which resulted from that phone hacking".
It was public revulsion over the revelation that Milly Dowler was targeted which forced Murdoch to shut down the 168-year-old News of the World in July 2011.
The court also heard that Brooks told Eimear Cook, ex-wife of golfer Colin Montgomerie, that phone hacking had been used for a story about former Beatle Paul McCartney.
"She said all you needed was a person's mobile phone number and a factory pin and you could listen to their voicemail," Edis said.
The trial heard that the News of the World even hacked the phones of journalists at the rival Mail on Sunday as part of a "dog-eat-dog" competition for stories.
"This was all about finding out how the competition were getting on with the story because, of course, you don't want to be scooped," Edis said.
The trial continues on Friday.