The Duke and Duchess of Cambridge have named their baby son Louis Arthur Charles.
The couple's third child, who was born in London on April 23 at 11:01am (local time), weighing 3.8kg (8lb 7oz), will be known as Prince Louis, Kensington Palace announced on Friday after a four-day wait.
"The Duke and Duchess of Cambridge are delighted to announce that they have named their son Louis Arthur Charles," the Palace said in a short statement.
"The baby will be known as His Royal Highness Prince Louis of Cambridge."
The baby is fifth in line to the throne and the Queen and the Duke of Edinburgh's sixth great-grandchild.
William and Kate's choice has come as a surprise as it is already one of Prince George's middle names. It also pays tribute to the Prince of Wales's great-uncle, Earl Mountbatten, who was murdered by the IRA.
Arthur had been a favourite with bookmakers for some time, with James and Philip also popular.
Alexander became a late front-runner after William remarked on the name at a Anzac Day service at London's Westminster Abbey while chatting with Australia's High Commissioner Alexander Downer, who asked William: "Have you thought of the name Alexander for him?"
"Well, it's funny you should say that. It's a good name," the duke replied.
Albert also entered the name race as the wait stretched on.
The middle name Charles is a tribute to the duke's father, the Prince of Wales.
The duke and duchess will have informed the Queen before asking the Palace to announce their son's name.
On Thursday evening at an event with Prince Harry, William said the couple were deciding on the name, telling reporters: "I'm working on that."
He also took a break from caring for the prince to carry out royal duties on Wednesday when he joined Harry and Meghan Markle at an Anzac Day service in London.
At the Anzac Day service the duke told guests that both Kate and baby were "in good form, luckily", and added: "Sleeping's going reasonably well so far, so he's behaving himself, which is good news."
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William and Kate's third child is a younger brother to Prince George and Princess Charlotte.
Four-year-old George was christened George Alexander Louis in 2013, while Charlotte, who turns three next week, was named Charlotte Elizabeth Diana in 2015.
Their names were announced when they were two days old.