Prince William and longtime girlfriend Kate Middleton are engaged and will marry next spring or summer in London — a royal wedding that Britons have been eagerly awaiting for years.

Prince Charles's Clarence House office said the second in line to the throne is "delighted to announce the engagement of Prince William to Miss Catherine Middleton."

The couple met more than eight years ago as students at the University of St. Andrews in Scotland and have had an on-again, off-again romance.

They got engaged last month during a vacation in Kenya.

The couple showed off the engagement ring that belonged to William's mother — Diana, the late Princess of Wales — amid the bright flash from cameras during a crowded news conference on Tuesday. The ring is an oval sapphire surrounded by diamonds.

Kate Middleton shows off her engagement ring, which belonged to the late Diana, Princess of Wales. (Paul Hackett/Reuters)

"It is very special to me. It was my way to make sure my mother did not miss out on today and the excitement that we are going to spend the rest of our lives together," William said.

William is second in line to the throne after Charles. A first child for William and Kate would move ahead of William's younger brother, Prince Harry, to become third in line to the throne.

Prime Minister David Cameron, told of the engagement during a cabinet meeting earlier in the day, said he was delighted and sent the couple his best wishes.

William once told an interviewer he wouldn't marry "until I'm at least 28 or maybe 30." He turned 28 in June and recently completed training as a Royal Air Force search-and-rescue pilot.

The Royal Family is said to approve of Middleton, the daughter of self-made millionaires. Her father, Michael, 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 farm in southern England.

Middleton works for the family business.

"[We] are absolutely delighted by today's announcement and thrilled at the prospect of a wedding sometime next year," Michael Middleton told reporters. "They make a lovely couple. They are very fun to be with and we have had a lot of laughs together."

Commentators have been anticipating a summer 2011 wedding, a date that avoids both the Olympics and the Queen's Diamond Jubilee, both in 2012. Bookmakers stopped taking bets on a 2011 wedding months ago, saying all signs were pointing to a marriage next year.

Earlier in November, the Middletons joined members of the Royal Family for a holiday at the Balmoral estate in Scotland — another milestone on Kate's road to acceptance by the family's inner circle.

The venue for the wedding has not been announced. "Prince William has informed the Queen and other close members of his family," the Clarence House statement said. "Prince William has also sought the permission of Miss Middleton's father."

The couple will "live in north Wales, where Prince William will continue to serve with the Royal Air Force," after the wedding.