Britain's Prince William and his wife, Kate, are expecting their third child together, Kensington Palace announced on Monday - and Twitter immediately lit up with reactions.

Within minutes of the pregnancy being announced, multitudes took to social media to congratulate the Duke and Duchess of Cambridge, with the hashtag "Royal Baby" trending worldwide.

British Prime Minister Theresa May tweeted: "This is fantastic news. Many congratulations to the Duke and Duchess of Cambridge."

Tweeted popular vlogger Louise Pentland, "Another #RoyalBaby!!! I'm so happy for them! I wonder if it'll be a boy or girl! I've got Baby Fever for all these new babies coming!!"

Tweeted Emmy-award winning reporter Erica Ricci, "Congratulations to #PrinceWilliam and #PrincessKate! The third time's the charm, as they say:)."

Kensington Palace announced the happy news in a statement released Monday morning.

"Their royal Highnesses The Duke and Duchess of Cambridge are very pleased to announce that The Duchess of Cambridge is expecting their third child," the palace statement read.

"The Queen and members of both families are delighted with the news," it added.

The royal couple already have two children: one son, George, and one daughter, Charlotte, ages 4 and 2, respectively.

The third child will become the fifth in line to the throne behind the 68-year-old Prince Charles, Prince William, Prince George and Princess Charlotte.

As with her two previous pregnancies, Kate, who is reportedly not yet past the 12-week mark -- is suffering from hyperemesis gravidarum, which involves nausea and vomiting more severe than the typical morning sickness many women suffer during early pregnancy.

The illness has forced Kate, 35, to cancel a planned engagement at the Hornsey Road Children's Centre in London, which had been due to take place on Monday morning. She is being cared for at Kensington Palace, the statement said.

The announcement also said Queen Elizabeth II and both families were delighted by the news. The child will be the 91-year-old queen's sixth great-grandchild.

The BBC's royal correspondent Nicholas Witchell said the announcement comes at the start of a "significant week" for the family "because Prince George is due to start at big school."

"Presumably his mother would be keen to take him to that, [but] whether she is going to be well enough to do that remains to be seen," he told BBC News.

"It had also been expected that the Duke and Duchess of Cambridge would be taking a foreign trip this autumn," he added.

"Whether they will be able to do that or whether the duchess will be well enough to do that also remains to be seen."