Prince William's wife Kate was admitted to hospital in London on Monday in the early stages of labour, Kensington Palace announced.
The Duchess of Cambridge is expecting her third child, after Prince George and Princess Charlotte. The baby will be fifth in line to the British throne.
The 36-year-old was admitted to the private Lindo Wing of St. Mary's Hospital in central London, where George, four, and two-year-old Charlotte were born.
She was accompanied by William, their Kensington Palace home said in a statement. The hospital is a mile (less than two kilometres) from the palace.
"Her Royal Highness The Duchess of Cambridge was admitted to St. Mary's Hospital, Paddington, London earlier this morning in the early stages of labour," the statement said.
"The Duchess travelled by car from Kensington Palace to the Lindo Wing at St. Mary's Hospital with The Duke of Cambridge."
Royal fanatics have been camped outside the hospital wing for several days in anticipation.
The baby prince or princess will be a sixth great-grandchild for Queen Elizabeth II, who turned 92 on Saturday, and her husband Prince Philip.
Kate is being cared for by consultant obstetrician Guy Thorpe-Beeston, who is the surgeon-gynaecologist to the royal household, and consultant gynaecologist Alan Farthing, the Queen's surgeon-gynaecologist — part of the trusted team who delivered George and Charlotte.