Andy Coulson, the former editor of the News of the World, authorised payment to a Royal police officer who he was told was asked to "nick" a phone directory containing private telephone numbers of members of the Royal household, the Old Bailey heard today.

The jury on day four of the hacking trial was shown a series of internal News International emails between Mr Coulson and his Royal editor Clive Goodman, in relation to payments to Buckingham Palace protection staff.

Mr Coulson and Mr Goodman deny charges of corrupting public officials by paying them for Royal telephone directories.

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In an email dated May 2005, Mr Goodman told Mr Coulson: "Andy - know your busy but…one of our palace cops has got hold of a rare and just printed Palace staff phone book. Every job, every name, every number. We usually pay £1,000 a go for these. It's a very risky document for him to nick. OK to put the credit through? It's one of our normal cash contributions only players."

When he received no reply, Goodman followed up with another request to which Mr Coulson replied: "Fine."

The jury was told that Mr Goodman is alleged to have paid a total of £1,750 for two palace directories. The Royal editor told Mr Coulson that cash payments were necessary to protect the identities of those officers selling the books. Without this safeguard Goodman told Coulson, his contacts would "end up on criminal charges, as could we".

News International's payment records named the two officers as "David Farish" and "Ian Anderson". The jury were told these were false names and the true identity of the officers has not been established.

Goodman, who was convicted along with Glenn Mulcaire of phone hacking in 2007, warned a senior executive on the NOTW that a "paper trail" of payments made to police officers could "put them, me, you, and the editor (Andy Coulson) in jail", the court was told.

The Royal editor was responding to management concerns about the high cost of cash payments to Goodman's sources. His email, shown to the jury, explained to the paper's deputy managing editor Paul Nicholas: "There are only three protected sources of mine that we pay in cash. One - who is less sensitive - is trying to set up an alternative secure method of payment that will not put him at risk. He's extremely valuable to me and I'd be grateful if we could cut him some slack for a few weeks to see what he can achieve. The other two sources are impossible to pay in any other way for reasons we've discussed . I'm not going to put it in writing but any paper or computer trail that leads to them or their families will put them, you, and the editor in jail." Goodman added that "without them" he would find it "very hard to do this job".

Andrew Edis QC, lead counsel for the prosecution, described the email exchanges involving Goodman and Mr Coulson as the "clearest possible evidence of the charges against them". Mr Edis linked the importance of the acquisition of the directories to phone hacking, saying "we know phone hacking was in full spate" in 2003.

The court was also shown an email from Goodman appealing to Mr Coulson over a payment methods to the phone-hacking specialist Mulcaire, who he referred to by the name "Matey". "There are costs for Matey in setting it up and maintaining it which he has to cover," he wrote. "A few weeks ago you asked me to find new ways of getting into the (Royal) family, especially William and Harry and I came up with this. It's safe, productive and cost effective and I'm confident it will become a big story goldmine for us if we let it run just a little longer."

Mr Edis said that Matey was Mulcaire and that Mr Coulson showed no sign of being confused by the email. "He didn't write back and say 'Clive, have you taken leave of your senses, what are you talking about?"

The case is expected to continue until Easter of next year.