Daniel Taylor, a lawyer whose firm, Taylor Hampton, represents Ms. Wightman, said it was extraordinary that the trial was going ahead at all. It is only the second such case to go to trial after dozens of cases, and more than decade of hacking-related litigation, he said, and it poses risks to both sides.

Harry, he said, “is likely to be subjected to rigorous and sustained cross-examination” by lawyers representing Mirror Group. That could prove embarrassing, given that the hacking took place during the years before Harry met his wife, Meghan, when he was single and involved in sometimes turbulent relationships.

But the Mirror Group is also taking a risk in going to trial, Mr. Taylor said, since a defeat could encourage other victims of hacking to come forward. “It is willing to do so because a win or partial success at trial will, in its view, slow or stop the oncoming tide of claims being brought against the company,” he said.

On Monday, Harry managed to irritate the judge, Timothy Fancourt, by not showing up for the trial as expected. Mr. Sherborne explained that he had stayed in California on Sunday to celebrate the second birthday of his daughter, Lilibet, and would be in court to testify on Tuesday and Wednesday. A lawyer for the Mirror Group expressed concern that he would not get a full day and a half for cross-examination.

Some royal watchers said it was a sign of Harry’s polarizing reputation in Britain that the media coverage before the trial focused on whether testifying would diminish his stature rather than on the journalism issues at stake.

“The kernel of what he’s trying to do is being watered down,” said Mr. Hunt, the former BBC correspondent. “It’s almost as if phone hacking is priced in,” he said, adding, “it wasn’t priced in if you were the victim.”