The prosecutor, Andrew Edis, said that evidence of the affair, which lasted from 1998 to 2004, showed that the two former editors of Rupert Murdoch’s tabloid The News of the World had a particularly close relationship, and that they would have been likely to have confided in each other.
Brooks edited The News of the World from 2000 to 2003, with Coulson serving as her deputy. He was upped to editor when she became editor of sister title The Sun.
Edis claimed the pair conspired to hack phones while working at The News of the World.
Edis said: “Mrs Brooks and Mr Coulson are charged with conspiracy, and when people are charged with conspiracy the first question the jury have to answer is: How well did they know each other? How much did they trust each other?”
Both defendants deny the charges against them.