Kate was taken to the private wing of St Mary's Hospital in west London just before 6 a.m. local time (0500 GMT) on Monday.

"The Duchess traveled by car from Kensington Palace to the Lindo Wing at St Mary's Hospital with the Duke of Cambridge (William)," said a statement from Kensington Palace, the couple's official residence. "Things are progressing as normal. It wasn't an emergency."

The sex of the baby remains unknown as the couple have said they want it to be a surprise.

The child would come behind its grandfather Prince Charles, and then William, in the line of succession to the throne. Any subsequent siblings would not affect the succession order after a recent change of the law by parliament ensured that male heirs no longer took precedence over female ones.

A traditional method is used to announce the birth. A messenger carries a piece of paper from the hospital to Buckingham Palace containing details of the gender, weight and time of birth.

The notice is then pinned on a board outside the main gates.

Kate, who was treated in hospital for acute morning sickness in the early stage of her pregnancy, made her last public appearance on June 15 at the "Trooping the Color" military ceremony for Queen Elizabeth's official birthday.

The Lindo Wing - where rooms cost about 5,000 pounds (5,800 euros; $7,800) - is where Princess Diana gave birth to William and his younger brother, Harry.

rc/mkg (AFP, AP, Reuters)