As lunchtime approached, he grew increasingly emphatic. “The sheer amount that was taken on Arthur has forced us to cut it to 2 to 1,” he said. “So, yeah, I have no idea why so many people are confident in Arthur.”

The participation of bookmakers at a royal birth has become a ritual.

By comparison to sports-betting pools, interest in names is not huge — amounting to roughly 100,000 individual bets, and about a quarter million pounds, or $350,000, said Jessica Bridge, the head of public relations for sports and news at Ladbrokes. The pool stands out because it is about 70 percent female and because the bets tend to be small ones, made by once-a-year betters who “enjoy a wee flutter on events that take over the entire nation,” Ms. Bridge said.

“And, of course, we love a good, happy news story, so it’s nice to feel a part of it somehow,” she added.

Lately, the news stories around betting in Britain have not been happy.

On Tuesday, The Times of London reported that the British treasury would reduce the maximum bet on fixed odds betting terminals from about $140 every 20 seconds to around $3, sending share prices in betting companies plummeting. The betting industry had lobbied vigorously to stave off a crackdown on the machines, which generated a profit of $2.5 billion in 2016.