The Duchess of Cambridge has been admitted to hospital and is in the early stages of labour, Kensington Palace has said.
She travelled by car from the palace to St Mary’s in Paddington, west London, with her husband the Duke of Cambridge.
The couple do not know the gender of their first child, who will be third in line to the throne.
The BBC’s Nick Witchell said there was likely to be no more news until the official announcement of the birth.
The world’s press have been camped outside St Mary’s for days in anticipation of the birth. The due date had never been officially announced but had widely been expected to be mid-July.
The Duke of Cambridge has been with his wife on annual leave and will have two weeks’ paternity leave.
Catherine’s final public appearance before the birth was at the Trooping the Colour ceremony in June.
Under new laws to succession the baby will remain third in line to the throne regardless of gender.
The Duke and Duchess of Cambridge were married at Westminster Abbey in April 2011, watched on television by millions of people around the world.
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