Britain's Prince William (R), the Duke of Cambridge, and Kate, the Duchess of Cambridge, arrive at the British Academy Film Awards (BAFTA) at Royal Albert Hall in London, Britain, on Feb. 12, 2017. (Xinhua/Han Yan)

LONDON, Sept. 5 (Xinhua) -- Queen Elizabeth's grandson Prince William and his wife Princess Catherine are expecting their third child, the royal couple announced Monday.

The couple's official spokesperson at their home, Kensington Palace in London, said Princess Catherine, the Duchess of Cambridge, had pulled out of an official engagement because of severe morning sickness.

In a brief statement posted on social media the palace said: "The Duke and Duchess of Cambridge are very pleased to announce that The Duchess of Cambridge is expecting their third child."

Royal observers in London said the couple had hoped to keep details of the pregnancy private for a longer period of time, but the princess's illness had meant the announcement being brought forward.

The duchess had been due to visit the Hornsey Children's Center in London later on Monday.

Her spokesperson said Princess Catherine was being cared for at Kensington Palace.

Queen Elizabeth was said by Kensington Palace delighted to hear that her grandson and his wife would soon have a third child joining their two other children Prince George, four, and Princess Charlotte who is aged two.

A message posted from Clarence House in London, home of the heir to the British throne, Prince Charles, said he and his wife the Duchess of Cornwall, were delighted with the news.

Prince William's younger brother Prince Harry, on an official visit to Manchester, said he was "very, very happy with the fantastic news".

Prime Minister Theresa May was among the first public figure to welcome the announcement, saying: "This is fantastic news. Many congratulations to the Duke and Duchess of Cambridge."

Media reports recalled that Princess Catherine, who is 35, suffered from hyperemesis gravidarum in her two previous pregnancies.

When the baby arrives he or she will become the fifth in line to the British throne, behind its grandfather Prince Charles, its father Prince William and siblings Prince George and Princess Charlotte.

The announcement sent social media sites into overdrive, with fans of the monarchy hailing the news.

Bookmakers William Hill offered odds of 33/1 for gamblers wanting a flutter that the Duchess will give birth to twins. Bets are also being taken on potential names, with Alice is the bookmakers' favorite if the baby is a girl, with James, Arthur and Phillip the favored boy's names.