LONDON (Reuters) - Britain s Prince William and his wife Kate are expecting their third child, their office said on Monday after the Duchess was forced to cancel an engagement due to severe morning sickness. Britain s monarchy has ridden a wave of public support in recent years due to the popularity of William, Kate, and William s brother Harry, and the news was soon making headlines around the world.  Queen Elizabeth, who is 91, said she was delighted.  The baby will be fifth in line to the British throne, after grandfather Prince Charles, father William and elder siblings George, 4, and Charlotte, 2.  The popular Harry becomes sixth in line to the throne.   Their Royal Highnesses The Duke and Duchess of Cambridge are very pleased to announce that The Duchess of Cambridge is expecting their third child,  Kensington Palace said.   The Queen and members of both families are delighted with the news.  The couple did not say when the baby was due.   Kate and William married in a lavish ceremony in 2011 watched by about two billion people around the world. Two years later the international camera crews and photographers camped outside a London hospital to record the birth of George, and returned two years later for his sister Charlotte. George and Charlotte have since appeared on the front covers of magazines around the world and traveled on official royal tours of Poland and Germany with their parents. George will attend his first day of primary school later this week.   This is fantastic news,  British Prime Minister Theresa May said.  Many congratulations to the Duke and Duchess of Cambridge.  William and Harry have been in the headlines in recent weeks as they led the efforts to remember their mother Princess Diana who died in a car crash in Paris 20 years ago last Thursday. In a sign of how the young royals have helped to modernize the House of Windsor, the princes have spoken about the trauma of her death and its lasting emotional impact. Harry was in Manchester visiting a center supporting people affected by a bombing at an Ariana Grande concert in May when he gave his reaction.  Fantastic, great, very very happy for them,  he said.  Kate last appeared with the princes on Wednesday when they visited a public garden at Kensington Palace dedicated to Diana but the palace said she would not attend an engagement at a children s center on Monday due to ill health.   As with her previous two pregnancies, The Duchess is suffering from hyperemesis gravidarum....,  the palace said.  The Duchess is being cared for at Kensington Palace.  Hyperemesis gravidarum can cause severe nausea and vomiting and requires supplementary hydration and nutrients. The condition forced Kate to be admitted to hospital in the early months of her first pregnancy with George. The couple have returned to live in Kensington Palace after an earlier spell in Norfolk, eastern England, enabling them to dedicate more time to their royal duties on behalf of William s  grandmother, the queen. William has also given up his job as an air ambulance helicopter pilot.  One of three herself, Kate had prompted speculation earlier this year that she wanted a third child when she was presented with a cuddly toy designed for newborns during a trip, telling William:  We will just have to have more babies.    