The BBC's Kelly Goodwin: "The horse may have been frightened by one of the cars"

The animal reared up near the Queen's carriage and one eyewitness said the Queen looked startled at the incident.

As the procession went towards the castle the horse reared again and the rider, who has not been named, was thrown to the ground.

The Queen with her guest, President Herzog

He was taken to the casualty unit at Wexham Park Hospital, Slough, and was treated for a shoulder injury and discharged.

The Queen is reported to have sent a message to the hospital asking after his condition.

The horse was unharmed.

Police said: "The incident did not delay the procession but did cause a short delay to the military band entering the castle."

One eyewitness was in tears as she described what she saw.

"The horse started playing up and it reared, but the rider managed to hang on and get it onto the pavement," said Pat Kent from south London.

"But it reared up again and hit the Queen's carriage and then the third carriage. I could see the Queen and it happened on her side. She looked very startled, but there was no panic.

"The poor young man seemed to be trapped under his horse after he fell, but the horse got up and seemed to be OK."

Once inside the castle, a special ceremony marked the start of a four-day state visit by Germany's president.