LONDON (AFP, REUTERS, THE WASHINGTON POST) - Britain's Prince Harry said he was thrilled after he and American actress Meghan Markle announced they were engaged on Monday, with the wedding due to take place in the spring of 2018.

Prince Harry, 33, currently fifth-in-line to the British throne, and Ms Markle, 36, became engaged earlier this month, the statement issued by Clarence House said.

"Thrilled, over the moon," said Prince Harry as he posed for photographs with his fiancee in the grounds of Kensington Palace in central London where the couple will live in a cottage.

Asked when he knew Ms Markle was "the one", he replied: "The very first time we met."

Ms Markle, who wore a white coat, said she was "so very happy"as she clutched her future husband's arm tightly during their brief appearance. They left with their arms around each other.

She showed off a dazzling three-stone ring, including two diamonds taken from the personal collection of his late mother Princess Diana. Designed by Prince Harry himself, the ring's centre is a diamond from Botswana, which his office said was a place special to the couple.

Earlier, a statement from Clarence House said: "Prince Harry has informed Her Majesty The Queen and other close members of his family. Prince Harry has also sought and received the blessing of Ms Markle's parents."

The couple met in July 2016 after they were introduced through friends. The prince publicly confirmed their relationship months later in a rebuke to the media, which had been intruding into Ms Markle's private life, but it was not until September that they made their first public appearance together at the Invictus Games in Toronto, a sports event for wounded veterans.

"We are incredibly happy for Meghan and Harry. Our daughter has always been a kind and loving person. To see her union with Harry, who shares the same qualities, is a source of great joy for us as parents," Ms Markle's parents Thomas Markle and Doria Ragland said in a statement.

We wish them a lifetime of happiness and are very excited for their future together."

The news drew congratulatory messages not just from Ms Markle's parents, but also from Queen Elizabeth II, Prince William and his wife Kate.

The couple will live at Nottingham Cottage in London's Kensington Palace, where Prince Harry's brother William and his pregnant wife Kate live with their two children.

The last time an American married into the royal family, Britain and the monarchy was plunged into crisis, the Washington Post reported.

In 1936, King Edward VIII abdicated the throne to marry the American socialite and divorcee Wallis Simpson.

In her acting career, Ms Markle is best known for playing Rachel Zane in the popular New York City legal drama Suits, which is filmed in Toronto, where Ms Markle has been living.

Like Prince William's wife Kate, Ms Markle will not become a princess in her own right after marrying Prince Harry, Reuters reported.

However, Prince Harry, like his brother, is likely to be made a duke when he marries, meaning Ms Markle would become a duchess.

She was born in Los Angeles in 1981. Her father was a TV lighting director for soaps and sitcoms and her mother a clinical therapist.

She made her first TV appearance in a 2002 episode of medical drama General Hospital and has appeared in a number of TV shows and films, such as Horrible Bosses.

In 2011, she married film producer Trevor Engelson, but they divorced two years later. She had her own lifestyle blog thetig.com, which she recently shut down, and she has become a prominent humanitarian campaigner, like Prince Harry.

Within seconds of the royal engagement being announced, Twitter erupted in celebration.

"Harry and Meghan" and #Royalengagement quickly began trending in the UK and worldwide as people offered their congratulations.

British Prime Minister Theresa May tweeted: "I would like to offer my very warmest congratulations to HRH Prince Harry and Meghan Markle upon their engagement. This is a time of huge celebration for two people in love. On behalf of myself, the government and the country, I wish them great happiness for the future."

Hours after the announcement, the pair made their first official appearance as an engaged couple in the Sunken Garden of Kensington Palace.

Asked if his proposal was romantic, Prince Harry replied: "Of course it was," before leaving with an arm around his fiancee, who showed off her diamond engagement ring to photographers.

"I sometimes still feel I am living in a goldfish bowl, but I now manage it better," he had said in an interview with US magazine Newsweek, published in June this year.

"But I am determined to have a relatively normal life, and if I am lucky enough to have children, they can have one too."