Royal officials announced that William will marry Kate Middleton next spring or summer in London, ending years of rumoured splits, reconciliations and will-they, won't-they speculation.
William is second in line to the British throne after Charles, his father. Kate and William's first child would move ahead of his younger brother Prince Harry to become third in line to the throne.
William, speaking in a joint TV interview, discussed marriage with Middleton for more than a year before he proposed during a vacation in Kenya last month.
"As every guy out there will know, it takes time a certain amount of motivation to get yourself going," William said. "It just felt really right out in Africa and was beautiful at the time."
He gave Middleton the engagement ring once worn by his late mother, Diana - an oval blue sapphire surrounded by diamonds from the jeweller Garrard.
"This was my way of making sure that my mother didn't miss out on today," William said as the couple posed for photographers in the state apartments at St. James' Palace.
Middleton acknowledged that being queen was "a daunting prospect." She declined to say whether the prince had proposed on bended knee.
Many in Britain welcomed the royal engagement as a rare piece of good news in a time of economic uncertainty and cutbacks - a time much like 1981, when millions watched Charles and Diana's fairy-tale wedding. Their marriage eventually ended in divorce - but no one was dwelling on that detail Tuesday.
William's grandmother, Queen Elizabeth II, and her husband Prince Philip "are absolutely delighted for them both," Buckingham Palace said. Prince Charles said he was "absolutely thrilled," and his wife, Camilla, duchess of Cornwall, said her stepson's engagement was "the most brilliant news."
"It's wicked," said the duchess, who had just attended an event at the theatre where the musical "Wicked" is playing.
Middleton's parents, Carole and Michael, welcomed the prince to their family.
"We all think he's wonderful, we're extremely fond of him," Michael Middleton said. "They make a lovely couple."
Prime Minister David Cameron wished the couple "great joy in their life together," and said when he announced the news during a Cabinet meeting it was greeted by cheers and "a great banging of the table."
Cameron, who said he had camped out on the street the night before Charles and Diana's wedding procession, predicted this royal wedding would be a "great moment for national celebration" that would unite Britain.
Charles' Clarence House office said he was "delighted to announce the engagement of Prince William to Miss Catherine Middleton." It used Twitter as well as a news release.
Few were surprised. Their engagement was the safest bet in Britain, an event so certain that bookies had stopped taking bets on a 2011 wedding. The date avoids London's Summer Olympics and the queen's Diamond Jubilee, both being held in 2012.
"Kate has been waiting for so long, I expected her to find someone else," said London tour guide Gabrielle Sullo, 53. "The media had called her 'Waitey Katie,' so it's about time that she stopping waiting."
No venue has been announced yet. For pomp, the ceremony is likely to fall between the extraordinary spectacle of the wedding of Charles and Diana in St. Paul's Cathedral and Charles' subdued second marriage to Camilla at Windsor Guildhall in 2005.
Patrick Jephson, Diana's former secretary, said her son's nuptials would be "a master class" in wedding planning.
The formal engagement is likely to turn the poised, brunette Middleton - already depicted approvingly in the fashion pages - into a global icon. With her confident good looks and long brown hair, Middleton has already become one of the most photographed women in Britain.
The palace will be hoping that she combines Diana's glamour and charm with a more commonsense approach to life. At 28, Middleton is considerably older than Diana was when she wed at 20 and has had greater life experiences and longer training in dealing with the media.
"She seems quite competent," said approving 22-year-old student Sarah Madden, "and seems to be just as wonderful as Diana."
William and Harry have spent a lifetime in the spotlight, with their drunken nights out and female friends the subject of constant tabloid gossip. William, who turned 28 in June, once told an interviewer he wouldn't marry "until I'm at least 28 or maybe 30." But since joining the military, both have kept a lower profile.
Middleton met William at the University of St. Andrews in Scotland. They shared a house along with other students in the seaside university town, where William initially studied art history before switching to geography.
In 2002, William paid 200 pounds to sit in the front row at a charity fashion show where Middleton was modelling in a daring outfit. They are thought to have started dating the next year.
St. Andrews congratulated the couple Tuesday, pointing out that the school has a reputation as "Britain's top matchmaking university."
A wealthy commoner rather than an aristocrat, Middleton is the daughter of self-made millionaires. Her father worked for an airline and her mother was a flight attendant before they started a mail-order business specializing in children's parties, run from their house in southern England.
She attended Marlborough College, an elite private school, where she played tennis and field hockey, before studying art history at St. Andrews. After graduating in 2005, Middleton worked as a buyer for the fashion chain Jigsaw. She is now employed by her family's party-planning business.
The couple's relationship became public with a joint photo on a Swiss skiing holiday in 2004. Middleton then became a media darling - especially after both graduated, which ended a British media agreement to leave William alone while he was at university.
Middleton was there when William was commissioned as a British Army officer after graduating from Sandhurst military college in 2006. She was photographed attending public events, going to work, even getting a parking ticket - a level of attention that evoked the romance of William's parents.
But William was determined that Middleton would not suffer the same media hounding endured by his mother, who died in a Paris car crash in 1997. He appealed through his office for the media to leave her alone.
In 2007, Middleton filed a harassment complaint against a British newspaper. She accepted an apology and admission of error from the Daily Mirror.
At the time, an engagement was so expected that the retail chain Woolworths even commissioned mugs, plates and other Wills-and-Kate memorabilia. The chain has since gone out of business.
Yet only weeks later in 2007, media reported - and Clarence House did not deny - that the couple had broken up. Newspapers pored over the apparent end of the relationship in long stories sourced to anonymous "friends."
William's army training kept them apart, said some. The media pressure was too much for her, said others. Still others murmured that senior courtiers felt Middleton's middle-class background wasn't royal material.
Soon, however, the same newspapers were reporting that the pair had rekindled their romance. They were photographed leaving a London nightclub together, and Middleton was snapped on a stag hunting expedition at the royal family's Balmoral estate in Scotland alongside Charles.
When William graduated from his first flying course in spring 2008, Middleton applauded from the sidelines - although his training was not without incident. The Ministry of Defence confirmed that William had landed a helicopter on Middleton's parents' lawn during a training flight and flew a Chinook to a friend's stag party on the Isle of Wight - earning him a drubbing in the press for his perceived sense of entitlement.
William later served a two-month deployment with the Royal Navy before training to become a Sea King search-and-rescue pilot with the Royal Air Force. He recently completed that training.
The pair have recently seen each other mostly on weekends, with William a frequent visitor to the Middleton family house in the affluent village of Bucklebury, 80km west of London.
Earlier this month, Middleton's parents were invited to join members of the royal family for a shooting holiday at Balmoral, another milestone on her road to acceptance into the royal family's inner circle.
Clarence House said after the wedding, the couple will live in north Wales, where William is based with the RAF.
Middleton has rarely, if ever, spoken about William in public. "I love the uniform. It's so, so sexy," - her assessment at William's graduation from Sandhurst - was a rare slip.
The couple were to give their first joint interview later Tuesday.
Not everyone was happy about the expected extravaganza. Graham Smith of the anti-monarchy group Republic said a lavish state-funded wedding amid a time of cutbacks was inappropriate.
"They need to pay for this event entirely themselves and not try to use it as some sort of PR exercise for the monarchy," Smith said.
