British royal couple Kate Middleton and Prince William are "very pleased to announce" that they are expecting their second child.
The Queen and members of both William and Kate’s families are “delighted” with the news, Kensington Palace said.
Ms Middleton 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 will no longer be joining her husband on a planned engagement in Oxford today.
British 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.”
Rumours that a second royal baby might not be too far off surfaced during the couple’s overseas tour to New Zealand and Australia.
When William was presented with a lace shawl for Prince George in Wellington in April, he told its creator Cynthia Read “You might have to make another one soon”, prompting the suggestion that a little brother or sister for George was on the cards.
Mrs Read revealed: “The way William said it was like he was dropping a hint, letting me in on a secret.”
But Kate was quick to quash the speculation by tasting wine at a vineyard in New Zealand soon after and telling wine-makers she was really enjoying being able to drink again after giving birth.
She also took part in a high-speed, white-knuckle jet-boat ride in Queenstown. The driver asked if anyone was pregnant during the safety talk beforehand. “No one put their hand up,” he disclosed.
During a formal welcome to Christchurch by the indigenous people of the local area, Henare Rakiihia Tau, chief of the Ngai Tuahuriri made the royal couple laugh by urging them to have another child and declaring: “May you do what princes and princesses have always done and increase your family.”
A sibling for George always appeared to be a question of when rather than if.
While on an overseas tour to Singapore in 2012, William was asked by a group of teenagers how many children he would like to have. He said he was “thinking about having two”.
William and Kate talked openly of having a family when they announced their engagement and they planned ahead by saving the top of their wedding cake - traditionally served at the christening of a first-born. The Duchess is also close to both her sister Pippa and brother James.
When George was born on July 22 2013, the Duke, in a statement on behalf of himself and the Duchess, proclaimed: ”We could not be happier.”
As they showed their new baby to the world for the first time on leaving hospital, Kate remarked that becoming parents was ”such a special time”.
In March, when George was just eight months old, William was questioned about increasing his family when he visited the Irish Guards at their military base in Aldershot with Kate on St Patrick’s Day.
Former regimental sergeant major Ray Collister asked if he was planning a brother or sister for George. The Duke replied: ”Maybe one day, one’s enough at the moment.”
Kate told a soldier at a parade the year before, when pregnant with George, that she was hoping for a boy and William a girl.
The new royal baby will be born fourth in line to the British throne.
As a sibling to Prince George, the Duke and Duchess of Cambridge’s second child will not be expected to be crowned sovereign.
But second-born royal children – often dubbed the “spare to heir” – have on occasion ended up as monarch.
The UK’s last king, George VI, was not meant to accede to the throne and only did so when his older brother Edward VIII abdicated over his love for American divorcee Wallis Simpson in 1936.
George VI’s father, George V, was also not destined to wear the crown. But he outlived his older brother the Duke of Clarence and Avondale – Prince Albert Victor – who died from flu in 1892. George V became king in 1910.
William and Kate’s new baby will be a great-grandchild to the Queen and a great-great-great-great-great-grandchild of Queen Victoria.
Once he or she arrives, Prince Harry will shift down the line of succession to fifth in line to the throne, while the Duke of York will move to sixth place and princesses Beatrice and Eugenie to seventh and eighth.
The baby will be a prince or princess thanks to the Queen, who stepped in ahead of Prince George’s birth to ensure all William’s children would become HRHs with fitting titles.
The Queen issued a Letters Patent under the Great Seal of the Realm in December 2012 when Kate was just a few months’ pregnant, declaring “all the children of the eldest son of the Prince of Wales should have and enjoy the style, title and attribute of royal highness with the titular dignity of prince or princess prefixed to their Christian names or with such other titles of honour”.
A Letters Patent in 1917, issued by George V, limited titles within the royal family, meaning a daughter born to William or Kate would not have been an HRH but Lady (forename) Mountbatten-Windsor instead and a second-born son would also have lacked the HRH title and become Lord (forename) Mountbatten-Windsor rather than a prince.
William’s cousin Princess Eugenie, who was born in 1990, was the last royal baby to be given the title Princess. The Earl and Countess of Wessex’s daughter Lady Louise Mountbatten-Windsor is also technically a princess, but her parents decided, with the Queen’s agreement, that she would use the courtesy title of the daughter of an Earl instead.
If the baby is a girl, it will be the first time a great granddaughter of a still-serving sovereign has been born in direct succession on the male line since 1897, when George VI’s sister Princess Mary was born.