US President Barack Obama and his wife Michelle spoke of their joy after Prince William's wife Kate gave birth to a baby girl on Saturday.

"Michelle and I are delighted to congratulate the Duke and Duchess of Cambridge, her Majesty the Queen and the royal family, and all the people of the United Kingdom on the birth of the Royal Princess," the White House said in a statement.

"On behalf of the American people, we wish the Duke and Duchess and their son George much joy and happiness on the occasion of the arrival of the newest member of their family."

News of the birth sparked celebrations in Britain.

The new Princess of Cambridge, who weighed in at 8 pounds 3 ounces (3.7 kilos), comes after big brother George in the line of succession.