Video report by ITV News Royal Editor Chris Ship
The Queen, Prince Charles and Prince William have all expressed their delight about the birth of the Duke and Duchess of Sussex's baby boy.
Speaking at an event at the Cutty Sark in London, just a day after Prince Harry and Meghan welcomed their firstborn child, his brother Prince William said he was "absolutely thrilled".
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."
Alongside the Duke was wife Kate, who also congratulated Prince Harry and Meghan on the birth of their son.
She said: "As William said, we're looking forward to meeting him and finding out what his name's going to be 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."
The Queen meanwhile was quizzed about the birth of her new great-grandchild - and her eighth in total.
At a luncheon for members of the Order of Merit, she was asked by a guest "life is good for Your Majesty?"
The smiling Queen replied: "Yes, thank you."
"Congratulations. Another great-grandchild!" she was told.
The Queen replied happily: "Yes, I know."
And Prince Charles, who was joined by his wife Camilla in Berlin, said: "We couldn't be more delighted at the news and we're looking forward to meeting the baby when we return."
Before a private meeting, the German president presented Charles with a gift for the new royal baby.
Frank-Walter Steinmeier gave the Prince of Wales a teddy bear wearing blue clothes, saying: "First of all, let me congratulate you."
The Prince has spoken about about his newly-born grandson at the Queen's Birthday Party held at the British Ambassador's residence in Berlin
He said: "It is also a particular pleasure to be back in Berlin once again - especially as the grandfather of a brand new grandson."
On Tuesday morning, the arrival of the Duke and Duchess of Sussex's baby son was celebrated during the Changing the Guard ceremony at Windsor Castle.
The Band of the Royal Regiment of Scotland, dressed in red tunics and bearskin hats, played the musical classic 'Congratulations' in the baby's honour.
Harry and Meghan have also asked the public to donate to four children's charities - Little Village, the Lunchbox Fund, Well Child, and Baby2Baby - instead of sending presents for the baby.
The new baby is seventh in line to the throne and the Queen's eighth great-grandchild.