After much waiting, the name of the new royal baby has been announced as Louis Arthur Charles.
The Duke and Duchess of Cambridge have chosen names with huge royal significance for their third born.
The name was announced by Kensington Palace shortly after 11am.
In a short statement the Palace said: "The Duke and Duchess of Cambridge are delighted to announce that they have named their son Louis Arthur Charles.
"The baby will be known as His Royal Highness Prince Louis of Cambridge."
Young Louis' name is believed to be a nod to the Prince of Wales's great-uncle, Earl Mountbatten, who was murdered by the IRA in1979.
Louis is already one of older brother Prince George's middle names.
The middle name Charles is a tribute to the duke's father, the Prince of Wales.
Bookmakers had Arthur as the favourite for some time. Arthur has been a popular middle name for royal sons during the Windsor era has not been common as a first name. One of Charles' middle names, it is also one of William's and was a middle name of the Queen's father, George VI.
The prince is fifth in line to the throne and the Queen and the Duke of Edinburgh's sixth great-grandchild. The duke and duchess would have informed the Queen before asking the Palace to announce their son's name.
Louis was the 71st most popular name in England and Wales, according to the latest available figures from the Office for National Statistics.
In 2016, just 911 boys were given the name Louis in England and Wales, but its popularity is likely to soar after getting the royal stamp of approval.
One of the first tributes paid to the little prince was to have a baby camel, born on the same day as Louis, named in his honour.
Blackpool Zoo tweeted: "As the world waited for the arrival of the royal baby, we were heralding our very own special delivery - a tiny, double humped Bactrian camel calf, born on Monday morning, has now been named in honour of the new Prince Louis!! #RoyalBabyName"
The three-day-old was born on April 23 - St George's Day, the patron saint of England - at 11.01am, weighing 8lbs 7oz, heavier than both Prince George and Princess Charlotte.
William joked on Monday that he had "thrice the worry" now as he took the prince home to Kensington Palace, after the baby's debut on the steps of the exclusive Lindo Wing maternity unit in London.
Speaking on Wednesday at an Anzac Day memorial service - his first official event since the birth of his new son - Prince William said that both Louis and the Duchess were "well", and that Louis was "behaving himself" and "sleeping reasonably well".
Despite this, the Duke did appear to have a mini-sleep during the service.
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.
The duchess is on maternity leave, but is expected to attend Harry and Ms Markle's wedding in Windsor on May 19.
Before Louis had even been named, he had been immortalised in Lego, along with the rest of his immediate family, at Legoland.
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