Bishop Cottrell said he was standing with the Queen waiting for the Bentley to pick her up.
He said: "He [the chauffeur] got into the car, put the key into the ignition and it made that kind of throaty rasping noise that cars make when they are not going to start.
"So he tried again and again it makes the noise... three or four times.
"At this point, really for a bit of fun, I just took a little step forward and made a blessing over the car, which got a laugh from the crowd.
"The chauffeur tried the key again and it started. So then there was a great round of applause."
Surprisingly, the bishop didn't make a song and dance about vehicular intercessory blessings.
"It is one of those nice little incidents," he said. "We all saw the funny side of it."