The Sun newspaper reported Saturday that the couple had reached an "amicable agreement" to separate. Sources confirmed the split to the Press Association news agency.
William's Clarence House office refused to comment, saying it did not discuss the prince's private life, but royal sources did not deny the report, tacitly acknowledging it was true.
The newspaper said the split was caused by the huge pressures on the young couple and by William's career in the army. The second in line to the throne graduated from Sandhurst military academy in December and is undergoing further training at an army base in rural England.
News of the break-up took many royal-watchers by surprise. It was widely thought the couple would soon announce their engagement; one bookmaker was so certain of a royal wedding it had stopped taking bets on it.
Ingrid Seward, editor of Majesty magazine, said the couple's relationship had reached an impasse.
"They can't go forward because William is in the army and he's dedicated the next few years of his life to that, so he's not in a position to get married," Seward said.
"They had lived together when they were at university, so in a way their relationship has become more difficult. They have seen a lot less of each other and are under a huge amount of pressure."
The prince, 24, and 25-year-old Middleton met as students at St. Andrew's University in Scotland in 2001 and had been dating since 2003.
Middleton has been the focus of intense media pressure for months as rumors spread that the couple would announce their engagement. She has frequently had to dodge paparazzi on her way to work as a retail accessories buyer, in scenes that evoked the romance of William's father, Prince Charles, and the then-Lady Diana Spencer a quarter-century ago.
Charles and Diana married at St. Paul's Cathedral in 1981, in a televised ceremony watched by millions around the world. They had two sons, William and Harry, but divorced in 1996 after admissions of adultery on both sides. Diana died in a car crash in August 1997 in Paris.
Last month, Middleton lodged a complaint of media harassment with Britain's press watchdog, the Press Complaints Commission. She settled the claim earlier this month following an apology and admission of error by the Daily Mirror newspaper.