The Duchess of Cambridge is expecting her third child, Kensington Palace has announced.
In a statement the palace said the duchess was suffering from severe morning sickness, as she did with her previous pregnancies, so a public engagement she had later on Monday had been cancelled.
The statement said: "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 Queen and members of both families are delighted with the news.
"As with her previous two pregnancies, the Duchess is suffering from Hyperemesis Gravidarum. "Her royal highness will no longer carry out her planned engagement at the Hornsey Road Children's Centre in London today.
"The Duchess is being cared for at Kensington Palace."
It was always thought that the Duke and Duchess of Cambridge would go on to have three children.
Kate is one of three and had a happy home life with her sister, Pippa Matthews, and brother, James Middleton, and is close to both of her siblings.
Here is the timeline leading up to the announcement of the Duke and Duchess of Cambridge's third pregnancy:
:: September 2001 - Prince William and Kate Middleton meet at St Andrews University.
:: Christmas 2003 - The pair reportedly become a couple after sharing a house together with two friends.
:: April 14 2007 - William and Kate are confirmed to have split.
:: June 2007 - Speculation mounts that the couple are back together after they are seen kissing and dancing together at an Army party.
:: July 1 2007 - Kate joins William at the Wembley Stadium Concert for Diana.
:: April 11 2008 - Kate appears at William's side at his graduation ceremony from the RAF, suggesting the couple's relationship is now permanent and has the approval of senior royals.
:: November 16 2010 - William and Kate's engagement is announced by Clarence House.
:: April 29 2011 - The pair marry at Westminster Abbey. William is made the Duke of Cambridge by the Queen and Kate becomes the Duchess of Cambridge.
:: September 12 2012 - While on an overseas tour to Singapore, William is asked by a group of teenagers how many children he would like to have. He reveals he is "thinking about having two".
:: December 3 2012 - Palace officials announce that the Duchess is pregnant and being treated in hospital for severe morning sickness, known as hyperemesis gravidarum.
:: December 6 2012 - The Duchess is discharged from hospital after three nights, telling reporters she is feeling "much better".
:: December 16 2012 - Kate makes her first public appearance since leaving hospital by presenting the BBC's Sports Personality of the Year award to cyclist Bradley Wiggins.
:: January 14 2013 - St James's Palace announces that the baby is due in July.
:: February 5 2013 - Kate and William take a "babymoon" on the Caribbean island of Mustique with the Middletons.
:: February 12 2013 - Italian gossip magazine Chi uses photos of Kate in a bikini on holiday. Other foreign publications follow suit.
:: June 15 2013 - The eight months pregnant Duchess makes her last public appearance before the birth at the Trooping the Colour parade.
:: July 22 2013 - Kate, with William at her side, is admitted to the private Lindo wing at St Mary's Hospital in Paddington, London, in the early stages of labour at around 6am. It is announced that the Duchess has given birth to a son, weighing 8lb 6oz, at 4.24pm.
:: July 23 2013 - The baby prince - a future king - makes his public debut in front of the world's media on the hospital steps.
:: July 24 2013 - The baby's name is revealed as Prince George Alexander Louis of Cambridge. The Queen meets her new great-grandson for the first time.
:: September 12 2013 - Kensington Palace announces that William, an RAF search and rescue pilot, is to leave the military after more than seven years of service. He enters a "transitional year" to focus on royal duties and charity work.
:: October 23 2013 - George is christened in the Chapel Royal in front of his close family and godparents who include Zara Phillips.
:: February 3 2014 - Kate and George go on holiday without William to the Caribbean island of Mustique - the Prince's first trip overseas.
:: March 7 2014 - William and Kate fly off for a week-long holiday to the Maldives leaving seven-and-a-half-month-old George behind with his grandparents and nanny.
:: March 20 2014 - George's new full-time, live-in Spanish nanny is revealed as Maria Teresa Turrion Borrallo, who trained at the prestigious Norland College.
:: April 7 2014 - The family begin a three-week official tour of New Zealand and Australia.
:: July 22 2014 - George celebrates his first birthday. William and Kate hold a party at Kensington Palace with guests including the Queen, Carole and Michael Middleton and Zara Phillips.
:: August 7 2014 - Kensington Palace announces that William is to train as an air ambulance pilot before taking up a full-time role with East Anglian Air Ambulance based at Cambridge Airport in the spring of 2015.
:: September 8 2014 - Kensington Palace announces that the Duke and Duchess are expecting their second child. Kate is again suffering from hyperemesis gravidarum.
:: October 21 2014 - Kate carries out her first official engagement since the news of her pregnancy was confirmed. During the Singaporean state visit she tells the president's wife: "I've been looking forward to getting out of the house, that's for sure."
:: December 7 2014 - William, Kate and her growing bump head to New York for the first time on an official visit.
:: February 26 2015 - The heavily pregnant Duchess stays behind in the UK with George as William goes on a solo official tour to Japan and China.
:: March 27 2015 - On her last official public engagements before the birth, Kate, in a fuchsia pink Mulberry coat, and William meet Baroness Lawrence at the Stephen Lawrence Centre and see projects run by the charity eXceL Project in London
The Duke tells people he cannot wait for the baby to be born and that his first child was a "life- changer" but "number two is a game-changer".
:: April 28 2015 - The Cambridges send pastries to fans waiting outside the Lindo Wing. Kate is seen driving herself away from Buckingham Palace after taking George there for a swim.
:: April 29 2015 - William and Kate celebrate their fourth wedding anniversary.
:: May 2 2015 - Kate is admitted to the Lindo wing and gives birth to a daughter, weighing 8lb 3oz at 8.34am.
:: May 4 2015 - The new princess's name is announced as Princess Charlotte Elizabeth Diana of Cambridge.
:: July 5 2015 - Charlotte is christened at the Church of St Mary Magdalene in Sandringham. Her godparents include William's cousin Laura Fellowes, a Middleton cousin of Kate's and the Duke's close friend Thomas van Straubenzee. Bookmakers are already accepting bets on a third child for William and Kate.
:: July 13 2015 - William begins his new role as an East Anglian Air Ambulance helicopter pilot and admits he is feeling nervous.
:: July 22 2015 - Prince George turns two.
:: October 20 2015 - The Duke and Duchess attend their first ever state banquet at Buckingham Palace during the Chinese state visit.
:: January 3 2016 - William reveals in a Prince's Trust documentary that fatherhood has made him more "emotional" and prone to welling up.
:: April 2016 - William and Kate leave the children in the UK with their nanny and head overseas for an official tour of India and Bhutan.
:: April 21, 2016 - The Queen's 90th birthday.
:: May 2, 2016 - Charlotte celebrates her first birthday.
:: June 11, 2016 - George and Charlotte join the Queen, William, Kate and the rest of the royals on the balcony after Trooping the Colour celebrating the monarch's official 90th birthday.
:: July 8, 2016 - George carries out his first official engagement in the UK when he visits the Royal International Air Tattoo with his parents.
:: July 22, 2016 - George turns three, with William admitting his son was "far too spoilt" on his birthday.
:: August 2016 - The Cambridges go on holiday to the south west of France.
:: September/October 2016 - The family head to Canada to carry out a successful royal tour. Charlotte is heard uttering her first words in public and George is spotted squashing his nose up against a window as he looks out of a seaplane.
:: December 25, 2016 - George and Charlotte join their parents at a Christmas Day church service for the first time, while staying at the Middletons in Berkshire.
:: January 20, 2017 - William confirms he is to become a full-time royal and leave his job as an air ambulance helicopter pilot in the summer, and be based in London where his children will be schooled.
:: March 24, 2017 - George is to join private Thomas's School in Battersea, Kensington Palace announces.
:: May 2, 2017 - Princess Charlotte turns two.
:: May 4, 2017 - An announcement that the Duke of Edinburgh is to retire from public duties in the autumn with the rest of the royals stepping in to help the Queen prompts speculation that William and Kate will stick to just two children.
:: May 20, 2017 - George and Charlotte are pageboy and bridesmaid at their aunt Pippa Middleton's wedding to James Matthews.
:: May 22, 2017 - Kate attends the Chelsea Flower Show.
:: June 12, 2017 - After not being seen in public for three weeks, the Duchess visits King's College Hospital to meet victims of the London Bridge terror attack and hospital staff.
:: July 17, 2017 - George and Charlotte join William and Kate on a tour of Poland and Germany.
The Duchess jokes about having a third child after being given a present designed for newborns while in Warsaw. She turns to the Duke, laughing, and says: "We will just have to have more babies."
:: August 30, 2017 - Kate, William and Prince Harry visit a memorial space at Kensington Palace planted with white foliage and flowers in honour of Diana, Princess of Wales.
:: September 5, 2017 - Kensington Palace says in a statement: "Their Royal Highnesses the Duke and Duchess of Cambridge are very pleased to announce that the Duchess of Cambridge is expecting their third child."