A former university lecturer who carried out online research about a criminal defendant while serving as a juror has been jailed for six months.

Theodora Dallas, 34, who taught psychology at the University of Bedfordshire, was found guilty of contempt of court by three high court judges, including Lord Judge, the lord chief justice.

The case highlights the growing problem courts face in ensuring jurors do not use the internet to investigate cases in an era when looking facts up on a computer seems an increasingly natural instinct.

Dallas, who conducted her research at home, was a juror in the trial at Luton crown court in July 2011. She told members of the jury what she had found out about the defendant, Barry Medlock, who was on trial for causing grievous bodily harm. The judge had to halt the case after discovering what had occurred, though Medlock was eventually retried and found guilty.

Dallas will serve three months and be on licence for the remainder of the term.

Contempt proceedings were launched by the attorney general, Dominic Grieve. The judges refused Dallas permission to appeal but she can nonetheless apply directly to challenge the decision.

The lord chief justice said Dallas, who was in court for the hearing, had deliberately disobeyed the trial judge's instructions not to search the internet and added: "The damage to the administration of justice is obvious."

Sentencing Dallas, he said: "Misuse of the internet by a juror is always a most serious irregularity and an effective custodial sentence is virtually inevitable."

Judge said her counsel, Charles Parry, had made a plea to the court to be merciful and impose a suspended sentence. Rejecting the plea, Judge said there was no sufficient basis for suspension.

He said other jurors "were obviously concerned to ensure that their responsibilities as jurors was properly discharged. It also demonstrates that they had fully understood the prohibition against use of internet ... Her conduct in visiting the internet repeatedly was directly contrary to her oath as a juror."

Dallas, already suspended by the university, resigned her post at the main campus in Luton last week. The court had previously heard that she was a woman of "good character", and that she had gained a degree in the UK, having arrived from Greece as a 19-year-old.

In a hearing last week, she said she had been checking the meaning of grievous bodily harm on the internet then added the word "Luton" to a search which produced a newspaper report mentioning that Medlock had previously faced an allegation of rape. It showed he had been acquitted of the charge and included information not disclosed during his trial.

In a written witness statement to the judges, Dallas admitted that "sometimes my grasp of English is not that good".

"I did not understand that I could make no search on the internet," Dallas explained. "I had no intention at all to prejudice the jury in any way. I had no intention to disobey what the judge said. I really apologise. I never thought it would cause such disruption."

The court heard that she had suffered depression. It emerged that another juror told an usher that Dallas had carried out internet research while jurors were considering their verdict.

Judge acknowledged that such cases posed a relatively novel dilemma for the courts. "We are trying to produce a simple formula for use," he said. "This is a relatively new area."

After the sentence was passed, Grieve said: "I take no pleasure in bringing such cases but they send an important message. By her action Ms Dallas halted a trial which was near completion and aside from the financial implications, her actions resulted in the victim in the case being forced to return to court and give evidence for a second time.

"There can be little doubt that repeated warnings were given to Ms Dallas and her fellow jurors as to the prohibition on conducting research into the case which they were trying. Only three weeks earlier the solicitor general prosecuted juror Joanne Fraill for discussing a trial on Facebook; a case mentioned by the judge in his directions to the jury."

The duties imposed on jurors are becoming more rigorous. A report for the Ministry of Justice in 2010 found that two out of three jurors do not fully understand the legal directions given to them by judges when they retire to consider their verdicts.

Jurors deemed to have misbehaved are regularly being given custodial sentences. Last month Matthew Banks, 19, was jailed for two weeks after he pretended to be sick and instead went to a West End musical in London when he was supposed to be serving on a Manchester jury.

Judge Martin Rudland called him "frivolous".

In June 2011, Joanne Fraill, 40, was given an eight-month jail term after becoming the first juror to be prosecuted for contempt of court for using the internet.

She admitted using Facebook to exchange messages with Jamie Sewart, 34, a defendant already acquitted in a multimillion-pound drug trial in Manchester in 2010, and carrying out research into another defendant while the jury was deliberating.