The Duke of Cambridge said he was "absolutely thrilled" about the birth of the Duke and Duchess of Sussex’s son.
And the dad-of-three couldn't resist making a 'dad joke' he quipped his brother Prince Harry would be joining "the sleep deprivation society that is parenting".
Prince William and Kate Middleton are at the Cutty Sark in Greenwich this afternoon - the day after the birth of their new nephew.
The Duke and Duchess are launching The King's Cup which will see the couple go head to head in a major sailing regatta in support of their charitable causes.
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But today all everyone wants to speak about is the as-yet-unnamed Baby Sussex - delivered shortly before dawn on Monday.
Asked how he was feeling, the Duke of Cambridge said: "Obviously thrilled, absolutely thrilled, and obviously looking forward to seeing them in the next few days when things have quietened down.
"I'm very pleased and glad to welcome my own brother into the sleep deprivation society that is parenting."
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Kate continued: "As William said, we're looking forward to meeting him and finding out what his name's going to be so so it's really exciting for both of them and we wish them all the best.
"These next few weeks are always a bit daunting the first time round so we wish them all the best."
Kate and William were greeted by two children from Child Bereavement UK, seven-year-old Eddie Rodbourne and Sefofo Agbeko, 11, as they arrived at the Cutty Sark.
A crowd of about 50 gathered and cheered as the couple chatted to outside.
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Today the Duchess of Cornwall, on a trip to Germany alongside Prince Charles, visited an outpatient clinic for victims of domestic violence.
During her visit, the Duchess was given a onesie with German art on it as well as a balloon with the words "It's a boy".
On receiving the gift for the new royal baby, Camilla said: "As soon as we return I will deliver it to them, direct from Germany. I'm sure they will be thrilled."
The boy will be a cousin and new playmate for the Duke and Duchess's three children Prince George, Princess Charlotte and Prince Louis, the latter of whom has just turned one.
For now new mum Meghan Markle is enjoying her privacy at Frogmore Cottage after the birth of the healthy baby boy.
The world is yet to get a glimpse of the new royal - who will not be a prince nor be entitled to be referred to as His Royal Highness, unless the Queen intervenes.
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In his announcement, Harry said on Monday: "How any woman does what they do is beyond comprehension but we're both absolutely thrilled and so grateful to all the love and support from everybody out there.
"It's been amazing so we just wanted to share this with everybody."
He went on to say: "As every father and parent will ever say, you know, your baby is absolutely amazing, but this little thing is absolutely to-die-for, so I'm just over the moon."
It is hoped the first picture of Baby Sussex will be released on Wednesday.
