Update: James suffered from a minor allergic reaction, according to reports. It is not known what caused the reaction but a royal spokeswoman stated,

Originally posted January 24th at 10 p.m.: Prince Edward, Earl of Wessex, 43, and his wife Sophie were seen taking their five-week-old son James Alexander Philip Theo Mountbatten-Windsor to Great Ormond Street Hospital in London today after he became ill. He is currently undergoing tests to identify the illness, with his 42-year-old mother by his side.

A royal spokesperson refused to comment, calling it a "private matter." However, a source reported that,

Source: Times Online