Three former News of the World employees have pleaded guilty to phone hacking charges, an Old Bailey jury was told this afternoon, as part of the opening of the trial of Rebekah Brooks, Andy Coulson, Brooks's husband and four other former employees of the now closed Sunday newspaper.

The crown prosecutor revealed that the individuals had already pleaded guilty at an earlier stage in proceedings, as he outlined to the court that News of the World was at the centre of three criminal conspiracies dating back to the year 2000, involving the two former editors.

Andrew Edis QC said that those pleading guilty were former News of the World news editors Neville Thurlbeck, Greg Miskiw and James Weatherup. The court also heard that the private investigator contracted by the newspaper to undertake the alleged hacking, Glenn Mulcaire, had pleaded guilty.

But the prosecuting counsel told the jury that journalism was not on trial. "There is no justification of any kind for journalists for getting involved in phone hacking. That is an intrusion into people's privacy which is against the law," Edis said. "The prosecution says that journalists are no more entitled to break the law than anyone else," he added.

Edis told the court that the criminal activity was discovered as a result of a police investigation into the paper in 2011 following the revelation that the telephone of murdered schoolgirl Milly Dowler "had been hacked by somebody acting on behalf of the News of the World".

The prosecuting counsel told the jury that "the events were very big at the time" but that they must put what they remember about them "out of their mind" and try the case on the evidence that they heard.

Edis also told them they did not have to remember everything they heard during his opening. "This is not a memory test, it's a long trial," he said referring to the estimated five to six months allotted for the case. "There are three types of criminal behaviour alleged here," Edis told the jury.

The jury heard that the first centred on alleged phone hacking conducted by a private investigator hired by the paper, Glenn Mulcaire, "who was very good finding out personal codes" which were used to access other people's voicemails remotely.

"Mr Mulcaire was very good indeed at getting the codes for people's phones and therefore able to get into other people's messages. It was very useful," Edis said.

He added that Mulcaire's activities helped the tabloid prove the truth of news under investigation such as affairs of people they were interesting in writing about.

The second conspiracy centred on paying "public officials, civil servants, police officers, soldiers" for confidential information, Edis said.

The third type of criminal behaviour related to an alleged conspiracy to pervert the course of justice when the investigation into the paper was ongoing in July 2011. Some of the people on trial, Edis said, were "secretly trying to prevent information coming to the attention of the police".

The crown opened its case to the jury of nine women and three men by identifying the location of the eight defendants in the dock at the back of court 12 starting with Ian Edmondson, the former head of news at the Sunday newspaper, who sat closest to the door.

Next to him was Brooks, the former chief executive of News International and a former editor of the News of the World and the Sun who is facing five charges spanning a decade linked to all three conspiracies – the conspiracy to intercept mobile telephone voice messages, a conspiracy to corrupt public officials by paying them for information and a conspiracy to pervert the course of justice. On her left sat Coulson, another former News of the World editor, who took on the job after Brooks.

Next to him was a seat for a fourth defendant Stuart Kuttner, the paper's former managing editor. Both Coulson and Kuttner are facing charges linked to a phone-hacking conspiracy.

The Sunday paper's former royal editor, Clive Goodman, was next in the row of eight, and faces a charge of conspiring to cause misconduct in public office by offering money to public officials for information.

Next to him in the dock was Rebekah Brooks's former personal assistant Cheryl Carter and her husband Charlie Brooks, who are facing charges linked to perverting the course of justice.

The final defendant is the then News International head of security, Mark Hanna, who also faces charges linked to perverting the course of justice.

Notebooks belonging to Rebekah Brooks, and computers and other material which could have been relevant to the phone hacking inquiry were hidden from police investigating it, the jury was told.

Material was removed from Brooks's London and Gloucestershire homes immediately before the News of the World was shut down in 2011 in the wake of allegations that Dowler's phone had been hacked, Edis said.

"It wasn't a secret that there was an investigation going on and by July of 2011 when the Milly Dowler allegation was being made, there was a great storm of publicity," Edis added.

The prosecution said there was "quite a complicated little operation" to hide material from the police who were investigating.

Edis said it was also alleged that Brooks asked Carter to remove her "journalistic notebooks" from the News International archive, where they were being stored.

"They were got out of the archive on the Friday before the last edition of the News of the World was published," he said, adding that the notebooks had "disappeared ... The police would have wanted to know what was in those notebooks."

Edis added that Brooks, her husband Charlie, and security official Hanna had "ensured that material the police would have wanted – computers, documents, things like that – were carried out of the Brooks's country home in Gloucestershire and taken to News International offices in Wapping".

"On their way or as part of the same operation, material was collected from their London flat and taken to the same place."

He said one might imagine that material would be "directly relevant to the police inquiry".

"Quite a complicated operation was set up to prevent that happening, which was discovered as a result of an accident which was rather bad luck for those conspirators involved." Edis said he would give more details later, but said that the circumstances were "quite memorable".

Earlier Edis said the prosecution case was that "people in charge of the purse strings" at the News of the World knew about phone hacking. "We say we will be able to show that there was phone hacking at the News of the World. That Glenn Mulcaire did it. That Clive Goodman did it, and that Ian Edmondson did it."

The case continues.