Kensington's Palace announced that Lady Cambridge, the wife of William's son, had a son.
The infant was born in St. Mary's Hospital in London, eight pounds of weight and seven ounces in length.
Britain's newest member of the family is the heir to the heights of the Kingdom.
Newly married princess Charlotte, princess of New Caribbean, won the fourth place in law in 2013, according to her legal changes.
Before the change was introduced, a woman could only have the right to crown unless the family lived.
Prince William, Katherine's first child, Prince George, was born in 2013 and the second, Charlotte, the second, family, was born in 2015.
Mother-in-law and the Baby Prince are in good health, according to the Kensington Palace.
The new Prince's name is due to be published later.
Several names have been proposed for the Infant Prince, and Arthur, Albert, Frederick, James and Philip are in the forefront.
British Prime Minister Theresa May, who sends a message of greetings to Prince William and Lady Catherine, said: "They were greeted for a brilliant future."
In this chart, the new crown is given to the British crown.
"I would like to warmly congratulate the Cambridge duke and the assailant on behalf of the London duchess for the birth of a son, son of a son." The message sent by the London Mayor Sadeq Khan shows,