Princess Eugenie and Jack Brooksbank's new baby will bump the King's brother Prince Edward down the line of succession.
The couple welcomed second son Ernest George Ronnie Brooksbank on May 30.
The royal mum posted two sweet snaps of the newborn on Instagram today, revealing he weighed 7lb 1oz.
She wrote: "Jack and I wanted to share the news that we had our little boy, Ernest George Ronnie Brooksbank on 30th May 2023 at 8.49 weighing 7.1lbs.
"He is named after his great great great Grandfather George, his Grandpa George and my Grandpa Ronald. Augie is loving being a big brother already."
The new arrival is 13th in line to the throne moving the Duke of Edinburgh, Prince Edward down to 14th place.
Ernest sits below his brother August Philip Hawke Brooksbank in 12th.
Eugenie and Jack's first born had initially been 11th when he was born in February 2021, before being bumped down the list by Prince Harry and Meghan's daughter Lilibet, who slotted into seventh.
Ernest is also a 13th great-grandchild of the late Queen - and is the first one to be born since her death last September.
He will not receive a title or HRH status as he is not a grandchild of the current monarch King Charles, who is his great uncle.
Eugenie, 33, who was heavily pregnant as she attended the Coronation on May 6 revealed she was expecting in January, both on Instagram and with an announcement from Buckingham Palace.
The palace said in a statement: "Princess Eugenie and Mr Jack Brooksbank are pleased to announce they are expecting their second child this summer.
"The family are delighted and August is very much looking forward to being a big brother."
On her personal Instagram account, Eugenie shared a photo of young son August kissing her bump.
In the photograph, taken by Jack, Eugenie, dressed in a woolly hat, beamed as August, in a burgundy winter hat, matching trousers and wellies, embraces her and kisses her bump, as they stand next to a fence in a leaf-strewn field.
She wrote: "We're so excited to share that there will be a new addition to our family this summer."
Eugenie's first born August Philip Hawke Brooksbank arrived at the exclusive Portland Hospital in central London on February 9, 2021.
