The High Court ruled today that articles published in The Sun and Daily Mirror following the arrest of a suspect by police investigating the killing of landscape architect Joanna Yeates were in contempt of court.
Judges said both tabloid newspapers had breached contempt laws in reports about Miss Yeates's landlord, Christopher Jefferies.
Attorney General Dominic Grieve had brought contempt proceedings and told judges that reports would have posed a "substantial risk" of prejudicing any trial Mr Jefferies might have faced.
Both newspapers had disputed Mr Grieve's allegations and denied contempt.
A panel of three judges - which includes Lord Judge, the Lord Chief Justice - is expected to announce penalties later today.
Judges had reserved judgment after listening to arguments from all sides at a hearing in London earlier this month.
Miss Yeates, 25, who lived in Clifton, Bristol, disappeared on December 17 2010 after going for Christmas drinks with colleagues. Her frozen body was found on a roadside verge in Failand, Somerset, on Christmas Day.
Mr Jefferies, a retired teacher in his 60s, was arrested on December 30. He was subsequently released without charge and was "entirely innocent of any involvement", Mr Grieve told judges.
Mr Grieve said his concerns related to articles in the Daily Mirror on December 31 and January 1 and in The Sun on January 1.
Judges were told that one Daily Mirror front page carried the headline "Jo Suspect is Peeping Tom" beneath a photograph of Mr Jefferies, and another front-page headline read "Was Killer Waiting In Jo's Flat?", with the sub-headings "Police seize bedding for tests" and "Landlord held until Tuesday" below.
The Sun's front-page headline read "Obsessed By Death" next to a photograph of Mr Jefferies and below the words "Jo Suspect 'Scared Kids"'.
Mr Grieve said material in the articles gave an "overall impression" that Mr Jefferies had a "propensity" to commit the kind of offences for which he had been arrested.
Lawyers representing Mirror Group Newspapers, publisher of the Daily Mirror, and News Group Newspapers, publisher of The Sun, said reports did not suggest Mr Jefferies had a "propensity" to murder and said jurors would not have been prejudiced.
They said memories would have faded by the time of any trial and jurors would have followed the directions of a trial judge.
A 33-year-old engineer has admitted killing Miss Yeates.
Dutchman Vincent Tabak has pleaded guilty to manslaughter but denies murder.
Tabak, who lived next door to Miss Yeates, is due to go on trial accused of murder at Bristol Crown Court in October.
The Attorney General said after the ruling: "I welcome today's judgment.
"While there was a great amount of speculation and copy relating to Mr Jefferies across much of the media, these three pieces of newspaper coverage were a different matter.
"They breached the Contempt of Court Act and the court has found that there was a risk of serious prejudice to any future trial.
"This prosecution is a reminder to the Press that the Contempt of Court Act applies from the time of arrest."
Earlier today at the High Court, Mr Jefferies accepted "substantial" undisclosed libel damages from eight newspapers over allegations made against him over the death of Miss Yeates.
The retired schoolmaster was not at court for the settlement of his actions against the publishers of the Sun, the Daily Mirror, the Sunday Mirror, the Daily Mail, the Daily Record, the Daily Express, the Daily Star and the Scotsman.
In a written ruling, Lord Judge, sitting with Lord Justice Thomas and Mr Justice Owen, said: "In our judgment the two publications in the Daily Mirror created substantial risks to the course of justice.
"They constituted contempt under the strict liability rule."
Lord Judge said: "The material in the two publications of the Daily Mirror is extreme."
He added: "True, it does not positively assert that Mr Jefferies was guilty of involvement in paedophile crimes, or the unsolved murder many years earlier.
"It is submitted that the articles were unflattering, suggesting that he was an eccentric loner. So they were.
"But they went very much further.
"It was asserted, in effect directly, that his standard of behaviour, so far as sexual matters were concerned was unacceptable, and he was linked to both the paedophile offences and the much earlier murder offence."
Continuing the judgment in relation to the Daily Mirror, Lord Judge said: "The juxtaposition of the photographs of two murdered women, together with the layout of the places where they died in proximity to Mr Jefferies' home, was stark.
"And in the context of the murder of Miss Yeates herself, the second article implied that Mr Jefferies was in a particularly convenient position, as her landlord, to have gained access to her premises to commit a murder, according to the article, committed by an intruder."
Turning to The Sun, Lord Judge said: "The articles in the one issue of The Sun were written and laid out in such a way that they would have conveyed to the reader of the front page and the two inside pages over which the stories were spread that he was a stalker, with an obsession with death, who let himself into the flats of other occupants of the building where Miss Yeates lived, and that he had an unhealthy interest in blonde young women."
He said: "Although the articles contain statement or words which could be said to have been favourable to Mr Jefferies, these were quite insufficient to counter the way in which the spread of the articles, and their content, associated Mr Jefferies with this murder."
Although their effect "is not as grave as that of two series of articles contained in the Mirror, the vilification of Mr Jefferies created a very serious risk that the preparation of his defence would be damaged".
He added: "At the time when this edition of The Sun was published it created substantial risks to the course of justice."
Lord Judge said there was no doubt about the identity of the man who killed Miss Yeates "or that Mr Jefferies is innocent of any involvement in it".
He added: "By way of emphasis, he is not simply presumed in law to be innocent of the killing.
"As a matter of fact and reality he is innocent.
"He is not facing trial, and will never face trial."
However, at the time the articles were published he was under arrest and proceedings against him were active.
"No one was to know that before very long he would be entirely exonerated. That feature makes this an unusual case."
Publication of articles which vilify a suspect under arrest "may deter or discourage witnesses from coming forward and providing information helpful to the suspect "which may help immediately to clear him of suspicion or enable his defence to be fully developed at trial".