Kensington Palace announced that the baby - weighing 8lbs 7oz - was born at about 11am, after Kate was taken by car before 6am to the private Lindo wing of St Mary's Hospital in Paddington, London.
The infant is little brother to Prince George and Princess Charlotte and is fifth in line to the throne.
The birth was announced via an easel bearing the official notice outside Buckingham palace today, and read: 'Hey Royal Highness the Duchess of Cambridge was safely delivered of a son at 11.01am today. Hey Royal Highness and her child are both doing well.'
As with previous births, a gathering of loyal royal followers are camped outside the hospital and are in full celebration mode as they hope to catch a first glimpse of the newest royal.
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Image: Instagram @kensingtonpalace