Judge Jonathan Seely at Chelmsford Crown Court said he had never seen anything like it in his career.

After realising her mistake the woman, who had followed the jurors into court, alerted the judge.

“Don’t worry, none of us noticed or noted it,” he said. “The person who should be there for some reason did not head down. There were 12 people sworn in as jurors yesterday, you were not one of them. There is no question of me discharging you, you’re not on this jury.

“What will happen is we will restart this case with the original jury. You can go back into the pool and you will be able to serve on another jury.”

After the jury was assembled again, Judge Seely told them: “Well, ladies and gentlemen, I am not sure in my 30 years working in the courts this has ever happened before.

“The lady who was sitting in your position was, of course, not a member of this jury and simply followed the crowd I suppose.

“I have discussed the situation with counsel and there is no harm and no problem in the sense this was at a very early stage of the case.”