LONDON -- Kensington Palace announced Monday that Prince William and his wife, the Duchess of Cambridge, are expecting their third child.

The announcement released in a statement said the queen was delighted by the news.

Officials say Prince William's wife, the former Kate Middleton, is suffering from Hyperemesis Gravidarum, a form of severe morning sickness, and would not carry out her engagements Monday.

William and Kate, who are both 35, already have two children, Prince George and Princess Charlotte. Prince George, now four-years-old, was to start his first day of school on Monday. His sister, Princess Charlotte, turned two in May.

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Kate suffered the same severe morning sickness when she carried her first two children, forcing the couple to make their baby news public perhaps earlier than they would have liked due to the necessity to cancel Kate's public events.

Their choice of school for Prince George, Thomas's Battersea in south London, indicated that the Cambridges were settling into their Kensington Palace apartment, having moved recently from their home in Norfolk, England, Anmer Hall.

The new baby will bump its Uncle Harry -- Prince Harry to the rest of the world -- out of the top-five in the royal line of succession. Once Prince William and Kate have their son or daughter, the line-up of presumptive British monarchs will take the following order:

Prince Charles of Wales Prince William, Duke of Cambridge Prince George of Cambridge Princess Charlotte of Cambridge William and Kate's third child Prince Henry (Harry) of Wales Prince Andrew, Duke of York Princess Beatrice of York Princess Eugenie of York Prince Edward, Earl of Wessex

No details were immediately available about when the third baby was due, but betting agencies were quick to start offering odds on possible names for the soon-to-be-born child in the House of Windsor.

Paddy Power offered 8-to-1 odds on Alice. Also popular was Diana, after Prince William's mother, particularly given the timing of the announcement.

Together with Prince Harry, William and Kate have taken a leading role in marking the 20th anniversary of Diana's death. Last week the royals toured the garden of their Kensington Palace home to remember the princes' mother and celebrate her contributions to their family and to the monarchy.

"Given the recent anniversary, there'll be plenty of interest in the name Diana if the baby is a girl," the agency said.

The royals have said from the time of their engagement that they wanted to have a family -- the only question was how many children they wished for. Royal watchers recorded every aside for clues as to what the plans might be.

When asked on a royal tour in Singapore in 2012 about how many children he wanted, William said he was "thinking about having two."

More recently, during a royal tour of Poland, Kate joked about a third child when given a cuddly toy designed to soothe tiny babies.

Kate thanked the well-wisher for the present and turned to William. "We will just have to have more babies," she said laughing.

Kate is the eldest of three siblings, and reportedly had a very happy childhood. William has a younger brother, Prince Harry.

The royal couple can also be seen as following in the footsteps of Queen Elizabeth II and Prince Philip, who have four children.