Phone hacking was "rife" at all three of Mirror Group Newspaper's (MGN) national titles by mid 1999 at the latest, the High Court has heard.
At a hearing to decide the amount of compensation to be awarded in eight representative cases, counsel David Sherborne said that the date when the practice first started at the Daily Mirror, the Sunday Mirror and the People was not known.
But, the evidence was that it was rife on the showbusiness desk at the Daily Mirror by mid 1999 and continued thereafter.
James Hipwell, a former Daily Mirror journalist who is due to give evidence during the two-week hearing before Mr Justice Mann in London, characterised it as being "endemic" at the newspaper.
Mr Sherborne, who is representing TV executive Alan Yentob, actress Sadie Frost, ex-footballer Paul Gascoigne, soap stars Lucy Taggart, Shane Richie and Shobna Gulati, flight attendant Lauren Alcorn and TV producer Robert Ashworth, said: "It is a reasonable inference that phone hacking was rife at all three of MGN's national titles at or around the same time, that is by mid-1999 at the latest."
Counsel said that MGN had admitted that all eight were the victims of voicemail interception and other unlawful methods of information gathering by journalists working for the three titles.
It also admitted that a number of stories would not have appeared but for the voicemail interception.
Seven of the claimants - Mr Yentob was the exception as there were no stories in relation to him - relied on a total of at least 109 published stories.
Mr Sherborne said that the wrongdoing complained of was carried out intentionally for cynical commercial reasons.
"The fact that MGN continued its phone hacking at such levels and over the course of so many years more than evidences its utility as a valuable source of information, particularly for publishing stories in its three most popular newspaper titles."
Futhermore, he added, it deliberately concealed its wrongdoing at the time as well as subsequently, and tried not to create any incriminating evidence. If any such evidence was created, steps would be taken to destroy it.
He told the judge: "Your task at the end of the day is to assess the likely extent of the wrongdoing in relation to each of the claimants and then to make an award."
Mr Sherborne said that the enormous amount of material in the case reflected the size and the scale of the unlawful activities taking place within the organisation.
The articles which appeared were the "tip of the iceberg" as, for every one published because of hacking, there was all sorts of underlying unlawful activity which did not produce anything which could be used - either because it was far too risky or was not of immediate interest to readers.
"It does not mean that voicemail interceptions which did not result in articles were not intrusive. Some of it was obviously way too sensitive even for these deeply insensitive journalists to publish."
He said that in 2000, and the years which followed, people used their mobile phones differently and voicemail was used like texting or email today for busy people to communicate information.
It contained very useful information for journalists and their picture desks who wanted to know everything about these people who sold newspapers in their millions - "what they were doing, where they were doing it and who they were doing it with".
Their voicemail charted the ups and downs of all their relationships, both personal and professional, and provided often very intimate pieces of information about their health, welfare and finances.
Mr Sherborne said examples included Frost attending AA meetings, Richie's financial problems, Ashworth's divorce or Taggart's relationship with actor Steve McFadden.
"Strangers were deliberately picking through this, sifting for things they could get away with publishing."
He said that "hordes" of journalists would have been discussing their messages, which would also have contained emotional private expressions of happiness or regret.
"No-one imagined that all this private information, this treasure-trove about just the sort of people who filled these newspapers in their millions, could be accessed and listened to in this way, let alone plundered as a source for stories.
"It's no wonder that the theft of this information and the appearance of it in these three different newspapers caused an enormous amount of anxiety among these eight.
"Al of them described the paranoia, feelings of distrust, and the suspicion that they were betrayed by people close to them, with all the inevitable damage caused to relationships, their friends and family."
Mr Sherborne said that he did not need to use hyperbole to describe the scale of the activity as the facts spoke for themselves.
MGN sought to portray the intercepted messages as trivial or anodyne, but that was unsustainable on the evidence of the call data and level of interception which took place.
"In one of the most competitive industries, why else would the practice of voicemail interception be so rife throughout MGN's three titles?
"Why would journalists who had to write stories, fill pages and meet deadlines, why would they in their scores have persistently accessed voicemail messages of these people and those close to them, day in day out, week in week out, month in month out, for years?
"And we have evidence of at least eight - and possibly 10 - years.
"Why on earth would they have kept using this method unless it was hugely productive in all sorts of ways? Not least because it was valuable in producing stories. It makes no sense."
Developing his opening argument, which will take more than two days to complete, from documents before the court, Mr Sherborne said that. although Mr Hipwell did not generally remember the names of the showbusiness desk's victims, he remembered that Mr Yentob was one of the principal targets - and that journalists sang an amended version of a Spike Milligan song when they hacked his messages.
Although no articles appeared about him, the journalist said that Mr Yentob's voicemails provided a particularly fruitful source of good stories.
While Mr Hipwell's evidence covered the period from 1998 to February 2000 at the Daily Mirror, that of Dan Evans - who is also to attend as a witness - covered the period from April 2003 to December 2004 at the Sunday Mirror.
Mr Sherborne said the upshot of his evidence was that, if you were a target on Mr Evans's regular list, he would definitely be hacking your voicemails twice a day - every morning and evening - and if there was something of interest, he would be following it up with much more hacking during the day.
All of the eight claimants, except for Mr Gascoigne, were on Mr Evans's regular list.
At the height of his work, he had about 100 targets that he was checking daily and he was making in the region of 250 phone-hacking calls every day.
Mr Sherborne said that phone hacking was not the only "dark art" practised, as journalists would also use private investigators.
In MGN's written argument, Matthew Nicklin QC said it published a public apology to all hacking victims and parent company Trinity Mirror had sent private letters of apology to the eight claimants.
"It has offered undertakings, and explained that it has already given full disclosure and cannot give more information as to the unlawful activities than it has done so far.
"But these proceedings are not, and cannot be, a surrogate public inquiry. Some of the answers the claimants seek are outside what could be achieved in these proceedings and are likely to be unattainable in any event.
"The full extent of the voicemail interception will never be precisely known and nor will it be possible to establish precisely what information was obtained in relation to any particular act of interception."
In none of the claims could it be said with any realism that the discovery of hacking had caused serious long-term, life-changing suffering like that in personal injury cases.
Nor could their hurt be equated with the kind of traumatic lengthy bullying suffered in harassment or discrimination cases.
Most of the claimants spoke of the revelation of the hacking as being a relief and assuaging their concern that people close to them were responsible for passing on private information.
Mr Nicklin said that, by the time these discoveries were made, the unlawful accessing of the claimants' information had long ceased.
"All the claimants know that their voicemail messages are now perfectly safe , and no private investigator employed by MGN will be snooping around their private information."
He added: "The claimants now face trial secure in the knowledge that MGN has admitted liability, and has also publicly and privately apologised to them and expressed regret at what certain of its former employees did in the past."
The hearing was adjourned until tomorrow.