The baby son of the Earl and Countess of Wessex is in hospital after falling ill with a "minor allergic reaction", Buckingham Palace has said.
James is Prince Edward and Sophie's second child
Sophie, 42, took five-week-old James, who is eighth in line to the throne, to Great Ormond Street hospital in London on Thursday afternoon.
A royal spokeswoman said: "It's not life-threatening and it's not serious."
The palace added that it was not yet known what caused the reaction in the infant or what his symptoms were.
BBC royal correspondent Peter Hunt said royal officials had described the medical treatment as "routine tests".
James, whose full name is James Alexander Philip Theo, is the couple's second child and was born on 17 December at Frimley Park hospital in Surrey, weighing 6lb and 2oz.
He takes the title of Viscount Severn and is the eighth grandchild of the Queen. He is expected to leave hospital shortly.
Prince Edward went on a tour of parts of Worcestershire on Friday, visiting Droitwich to see the areas affected by flooding last year.
He also opened the South Bromsgrove High School in Bromsgrove, which has had state-of-the-art facilities worth £18m built.
On the subject of James's illness, headmaster Paul Copeland said: "The prince said that was something that he wasn't particularly worried about.
"He was taking the line it was being played up and it wasn't to be worried about.
"From our point of view, we thought he might not come but when we saw him arrive, we thought that was fantastic."
Prince Edward and Sophie also have a four-year-old daughter, Lady Louise Windsor.
In 2002 the countess lost her first child when she suffered an ectopic pregnancy.