Kate and William, the duke and duchess of Cambridge
"could not be happier" over the birth of their son, the duke said in
a statement late Monday.
Britain's future king weighed 8 pounds and 6 ounces (3.769 kilograms), a royal statement said, noting that William was present for the birth.
Kate and the newborn, who will be known as the prince of Cambridge, were to stay overnight in the private Lindo wing of St Mary's Hospital in central London.
Queen Elizabeth II and other senior members of the royal family were "delighted" with the news, the royal statement said. A name would be announced "in due course."
The baby is third-in-line to the throne, after his grandfather Prince Charles and father Prince William.
In keeping with tradition, a formal announcement was pinned to an easel just inside the gates of Buckingham Palace reading: "Her Royal Highness, The Duchess of Cambridge, was safely delivered of a son at 4:24 pm (1524 GMT). Her Royal Highness and her child are both doing well."
Prince Charles said he was "overjoyed" at the arrival of his first grandchild.
"Grandparenthood is a unique moment in anyone's life, as countless kind people have told me in recent months, so I am enormously proud and happy to be a grandfather for the first time, and we are eagerly looking forward to seeing the baby in the near future," he said.
Prime Minister David Cameron said the "wonderful news" was an "important moment in the life of our nation" and that of a "warm and loving couple."
"It has been a remarkable few years for our royal family," he said. "A royal wedding that captured people's hearts, that extraordinary and magnificent jubilee, and now this royal birth - all from a family that has given this nation so much incredible service."
US President Barack Obama and his wife Michelle congratulated the couple on the "joyous occasion" and wished them "all the happiness and blessings parenthood brings."
A girl would have been the first to automatically become queen even if she later got a younger brother, due to a recent change in the succession laws.
"The fact that it is a boy postpones the idea of authentic female succession," historian David Starkey said. "It means that the effect of the act that everybody has been labouring over will not now be felt for 100 years."
The birth of the prince means the queen has three living heirs, the first time that has happened since 1894, when Queen Victoria's great-grandson, the future Edward VIII, was born.
Reports that Kate had arrived at the hospital early Monday were confirmed by the palace at 7:30 am.
The late Princess Diana gave birth to both William and his brother, Prince Harry, in the same hospital.
Hundreds of reporters, camera operators, photographers and fans had been camped outside St Mary's doorway since the beginning of July, enduring England's first prolonged heatwave since 2006.
Thousands gathered Monday outside Buckingham Palace, where the queen arrived in the late afternoon, to get a glimpse of the official announcement.
Kate was attended by a top medical team, including the queen's former gynaecologist Marcus Setchell. As he left the hospital after the birth, he said simply: "wonderful baby, beautiful baby."
Two separate royal gun salutes are to take place Tuesday to celebrate the birth of the third-in-line to the throne.
The public could be kept waiting for news of the baby's name. William's parents took a week to announce his, while Charles' name was not known for a month.
"Prince William can choose any name he likes, but I suspect the names will be fairly traditional because this is going to be a future king," royal biographer Hugo Vickers said.
George and James are the bookmakers' favourites.
"George was the second most popular name before we knew whether the baby would be a boy or girl, so now it moves into pole position and is the red-hot favourite to be the name of the new prince," said a William Hill spokesman.
Henry, Prince Harry's real name, Albert, Arthur, Philip, Louis and Alexander are thought to be in the running.
William is expected to take two weeks paternity leave from his job as a British Air Force search and rescue pilot in Anglesey, North Wales, while it is not known how much time Kate is to take off from her royal duties.
The newborn is the queen's third great-grandchild and, all going well, will be Britain's 43rd sovereign since William the Conquerer took the throne in 1066.
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