In what is believed to be the first decision of its kind in more than three centuries, Judge Anura Cooray split legal opinion after he sentenced Bonnie Schot, 20, and Carol Barclay, 32, to 30 days for contempt of court on Monday after they cited "personal reasons" for not participating in the jury's deliberations in a counterfeit currency trial at Knightsbridge Crown Court in central London.
The pounds 100,000 17-day prosecution against five defendants, which was followed by a four-hour summing up, had to be abandoned and a new one ordered at an additional cost of pounds 150,000.
Ms Schot, the jury foreman, who had been planning to study law, later said from Holloway jail in north London: "There wasn't enough evidence for me to reach a decision ... I just didn't understand it."
As penal groups castigated what they said was an indefensible use of imprisonment, the 61-year-old judge became the latest judicial figure to resort to issuing a statement through the Lord Chancellor's Department, to defend his actions.
The pair were freed on bail yesterday, pending an appeal. Asked outside prison what she thought of the judge, Ms Schot said: "He's a very spiteful and vindictive man."
Paul Cavadino, chairman of the Penal Affairs Consortium, warned: "If jurors who genuinely do not understand the evidence in a complicated trial are pressured into bringing in a verdict, this is likely to produce unjust convictions or wrongful acquittals.
"This unreasonably harsh action can only discourage people from serving as jurors."
It took "moral courage" for a juror to admit he or she did not understand the evidence rather than going along with the prevailing view in the jury room, he added.
People with personal or other difficulties, as the judge made clear in this case, are allowed under the regulations to make these known to the court at the time the jury is selected.
Judges generally look on such representations sympathetically in the interests of convening a jury that will stay the course.
Leroy Redhead, the barrister who is representing the two women, said that Ms Barclay had told the court that she could not ethically judge anyone and find them guilty or not guilty, despite having sworn to reach a verdict according to the evidence at the start of the trial.
The judge said in his statement that the jury "knew full well that they had ample means available to them to inform the court immediately if they were confronted with any difficulties".
He added: "I was satisfied that [the women's] refusal to participate in the jury's deliberations constituted a clear contempt of court."
However, not all commentators condemned Judge Cooray's action. A spokesman for the Law Society, the solicitors' professional body, said: "The jury system is a vital part of our system of justice.
"If you do go on a jury, you have responsibilities which shouldn't be taken lightly. Simply opting out is not acceptable."
The controversy comes when the jury system is already under fire for its alleged inability to cope with complicated fraud cases and for its expense.
If it were re-elected, a Conservative government would take steps to remove thousands of cases from the system altogether and have them heard summarily by magistrates.