The Duke and Duchess of Cambridge are expecting their second baby.
The Queen and members of both William and Kate's families are "delighted" with the news, Kensington Palace said.
Kate is said to be suffering from acute morning sickness, as she did with her first pregnancy, and is being treated by doctors at Kensington Palace.
She is no longer joining her husband on a planned engagement in Oxford today.
Prime Minister David Cameron offered his congratulations to the couple today, saying: "Many congratulations to the Duke and Duchess of Cambridge. I'm delighted by the happy news that they're expecting another baby."
Kate was due to join William in formally opening Oxford University's £21 million centre dedicated to the study of China. He will still attend, as planned, Kensington Palace said.
The announcement that the couple are expecting their second child comes two months after Prince George turned one.
The third-in-line to the throne was born on July 22 last year to a worldwide fanfare.
Kate suffered with hyperemesis gravidarum with her last pregnancy and is suffering from the very acute morning sickness again, meaning she may need extra hydration, medication and nutrients.
The couple's second child will become fourth in line to the throne, shifting Prince Harry further down the line of succession.
The Duchess of Cambridge's announcement will throw into doubt whether she will be able to attend her first solo overseas tour, set to take place in Malta later this month.