Former News International chief executive Rebekah Brooks and ex-News of the World editor Andy Coulson had an affair for at least six years, a jury has heard
Prosecutor Andrew Edis told jurors at the Old Bailey that in February 2004, when the pair were both working at the media giant, they had been having a relationship for some years.
He said a letter was found on Mrs Brooks's computer from February 2004, that made the relationship clear.
Mr Edis said: "The point that I'm going to make in relation to that letter is that over the relevant period, what Mr Coulson knew, Mrs Brooks knew too. And what Mrs Brooks knew, Mr Coulson knew too - that's the point.
"Because it is clear from that letter that, as of February 2004, they had been having an affair which had lasted at least six years."
Mr Edis told the court that the pair had been having an affair dating back to around 1998, spanning the period covered by their phone-hacking conspiracy charge.
The court heard that the letter - apparently written by Brooks in response to Coulson trying to end the affair - included a declaration of her love for her colleague.
Mr Edis told jurors he was not revealing the affair to deliberately intrude into their privacy or to make a "moral judgment".
"But 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."
He said the revelation was likely to attract a "great deal of publicity" and may draw some "unfair, unkind and unnecessary" comment.
"I'm frightened to be without you" - Rebekah Brooks' love letter to Andy Coulson
"Finally and the least of our worries, but how do we really work this new relationship? There are a hundred things that have happened since Saturday night that I would normally share with you..some important, most trivial.
"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 really not sure how I will cope.
"I'm frightened to be without you... but bearing in mind 'the rules' you will not know how I am doing and visa versa. The thought of finding out anything about you or your life from someone else fills me with absolute dread.
"Also you said I had to email you if anything important happened..like if I was ill? I don't understand this... we are either there for each other or not surely?
"But for example, how does this work thing manifest itself. Do we limit contact until we absolutely have to... like leaving our execs to sort run of the mill joint stuff?
"I don't want to get this wrong. I hope that I've managed to put your mind at rest about Les..and that you two now have a better relationship. On KRM (Keith Rupert Murdoch), well he's not bollocking you must not brood on lack of calls.
"Obviously I can't discuss my worries, concerns, problems at work with you anymore... and visa versa... but I'll assume unless I hear different that we keep our professional relationship to the minimum... and avoid if possible without it being in any way awkward.
"If it is necessary or more importantly right that we two editors should deal with it, then we will.
"If either of us feels that we are not striking this balance then we must say...??
"Anyway, that really isn't where I am confused. I know what horror it means and I know why we have to stick to it."