The News of the World was paying a rival newspaper executive for information, the phone-hacking trial has heard.
In an email, former royal editor Clive Goodman warned his boss managing editor Stuart Kuttner that if payments to some of his “uniform” contacts became known, they would all be sent to prison.
The email was forward to Kuttner’s former PA Beverley Stokes outlining why these three people should only be paid cash.
The email, of July 2005, read: "Morning, Stuart. Understand that, as you know, there are only three people I ever pay in cash.
"Two are in uniform and we – them, you, me, the editor – would all end up in jail if anyone traced their payments. They've had Special Branch crawling all over them since we ran a five-par story about an Operation Trident arrest at Clarence House.
"Thanks to the way we pay them, they're untraceable.
"The third is an executive at another newspaper who is also taking on potentially life-altering risks for us and will not accept any other form of payment."
Goodman discussed the arrangement of payments with Stokes, the trial heard.
He referred, in one message of April 2006, to payments that needed to be made to "Mr Alexander" – a codename used for private investigator Glenn Mulcaire.
The court has already heard that former AFC Wimbledon striker Mulcaire has admitted hacking phones.
Goodman told Stokes that Alexander was "the most important in terms of the contact" and he was hoping to get a story about Prince Harry from him.
The email read: "I'm relying heavily on him to work his magic over Harry's passing-out party."
When reassured by Stokes that the payments were going to be made, Goodman told the PA: "Fantastic. I won't be found in the Thames wearing concrete wellies tonight."
He added in a further email to her: "As long as they're useful, I don't ask many questions about their Sunday school records."
Former News International chief executive Rebekah Brooks, 45, of Churchill, Oxfordshire; ex News of the World editor Andy Coulson, also 45, from Charing in Kent; former NoW head of news Ian Edmondson, 44, from Raynes Park, south west London; and the tabloid's ex-managing editor Stuart Kuttner, 73, from Woodford Green, Essex, are all on trial accused of conspiring with others to hack phones between 3 October 2000 and 9 August 2006.
Former NoW and Sun editor Brooks is also accused of two counts of conspiring with others to commit misconduct in public office – one between 1 January 2004 and 31 January 2012 and the other between 9 February 2006 and 16 October 2008 – linked to alleged inappropriate payments to public officials.
She faces another two allegations of conspiracy to pervert the course of justice – one with her former personal assistant Cheryl Carter, 49, from Chelmsford in Essex, between 6 July and 9 July 2011; and a second with her husband, Charles Brooks, and former head of security at News International, Mark Hanna, and others between 15 July and 19 July 2011.
Coulson is also facing two allegations that he conspired with former NoW royal editor Clive Goodman, 56, from Addlestone in Surrey, and other unknown people to commit misconduct in public office – between 31 August 2002 and 31 January 2003, and between 31 January and 3 June 2005.
All of the accused deny all of the charges.
The trial continues.