I have learned that the News of the World was apparently eavesdropping on the phone messages of Rebekah Wade, who at the time was the editor of its sister paper, the Sun (she still is - although she will soon become chief executive of the Sun's parent, News International).

She was one of 75 individuals identified by police as having their phone messages monitored by the private investigator, Glenn Mulcaire - who was jailed in 2007 for phone hacking, together with Clive Goodman, the News of the World's royal reporter.

The police informed her that she was on Mulcaire's list of those whose mobile-phone voicemails were being tracked and was asked whether she wanted to press charges. She declined.

It's not unusual for newspapers to spy on each other, even newspapers within the same organisation.

The disclosure may be particularly embarrassing for Andy Coulson, who was then editor of the News of the World and is now the director of communications for the Conservative Party.

He denies that he knew that Mr Goodman was hacking into the mobile phones of celebrities, politicians and others.

Update, 10:45: For anyone running more-or-less any substantial news organisation, it'll be difficult to know whether to welcome or dread the Guardian's investigation into the use of allegedly improper techniques by News International to obtain private information about individuals.

Some will see it as an incentive for journalists to clean up the way they carry out investigations.

Others will fear that it will restrict the ability of journalists to uncover genuine wrongdoing, that legitimate investigations will become harder as a result of apparent misbehaviour by those hacks pursuing celebrity tittle-tattle.

But over-riding all other thought and emotions will be one terrifying question: "is my news organisation going to be seriously tainted by this?"

Because more-or-less every newspaper employed journalists whose specific skill was to obtain private phone records, or ex-directory telephone numbers or other confidential personal information.

And these specialist hacks in turn got hold of the valuable data through their relationships with private investigators.

A good deal of this trade in personal confidential information has already been exposed by Richard Thomas, who has just retired as information commissioner.

In a series of reports and in evidence to the House of Commons culture, media and sport committee, he made a series of disclosures about newspaper activities that he regarded as "prima facie" illegal.

Here's a statement from him to MPs that he gave in March 2007, which refers to the results of an investigation he carried out into the business relationship between the press and a firm of private investigators (the investigation was given the codename Operation Motorman):

"The first thing I would need to share is that the 3,000 or 4,000 transactions identified... came from a total of 13,000 transactions in this one operation alone. We were careful only to put forward those where there was some sort of hard evidence of the transaction being positively identified as involving a journalist for a newspaper".

And this is what he cited as the evidence of payments being made by journalists for the information:

"We did have, and we do have still, the statements, the bank statements, the invoices - some of these well-known proprietors were including information such as 'payment for confidential information', payment for 'blagging' [obtaining information by deception] in some cases - so there was what I might call hard 'prima facie' evidence."

But although successful prosecutions were brought against the detectives (who were given a conditional discharge), there were no charges brought against journalists - because Thomas was advised that, in the climate of the time, the courts would not wish to punish journalists, even if there were evidence of wrongdoing.

However the degree of detail obtained by him about this trade was startling.

He said the market price for obtaining the phone records of an individual was £750. Criminal records could be had for £500. The name of the owner of a car cost up to £200. And to break the barrier of secrecy of the ex-directory phone system cost up to £75 per number.

Where did the money end up? Well, a flow-chart produced by the office of the information commissioner shows the press employing private detectives who in turn deal with phone companies, call centres, the DVLA and what's described as "police source".

Also striking is Richard Thomas's list of the most enthusiastic customers of the particular detectives under investigation [pdf link].

The Daily Mail was listed as the top customer, with 952 transactions "positively indentified" (in the words of a report by the Information Commissioner's Office).

Then came the Sunday People, with 802, and the Daily Mirror with 681 trades.

The Mail on Sunday was in fourth spot with 266 deals. And the News of the World was one place below, with 182 transactions.

Even the Observer, sister paper of the Guardian, was a customer - with 103 transactions.

I would imagine none of those papers will be watching the current humiliation of the News of the World and its owner, News International, with much relish.

Update, 13:10: To state the obvious, my story says that the News of the World appears to have been eavesdropping on Rebekah Wade, editor of its sister paper, the Sun.

And - according to my sources - that is what she believes.

However, it has been brought to my attention, by a Tory spokesman and some commenters here, that on 29 November 2006 the Guardian reported that Mulcaire had intercepted her voicemail messages.

So the Conservatives are saying this is old news.

But the original Guardian story also says that no-one at the News of the World had a clue that there was eavesdropping of Ms Wade. There's an implication in that Guardian article that Mr Mulcaire may have been tracking the messages she received for someone else.

So, to repeat, what I'm saying is that News International is convinced Mr Mulcaire was operating for the News of the World - though I am not saying that the editor of the News of the World, Andy Coulson, knew this was going on.

Sorry if this is a bit complicated.

