There had been speculation about an engagement announcement
Prince William has split from his girlfriend Kate Middleton.
The couple had been under intense media pressure since they met at St Andrews University, Fife, in 2001, BBC royal correspondent Peter Hunt said.
The prince's move to an army camp in Dorset, and Miss Middleton continuing her life in London, are also said to have put a strain on the relationship.
Clarence House has not commented on the split. There had been speculation about a possible engagement in recent months.
BBC royal correspondent Nicholas Witchell said: "It's a surprise, because it had seemed very stable and very steady."
He said he thought the prince's move to Bovington Camp in Dorset had "a lot to do with the final stages" of the break-up.
The Sun reported that the couple had seen each other no more than once a week since then.
"One must suspect, though I don't know that this is absolutely certain, that it is more his decision than hers," said Nicholas Witchell.
"He has said in the past that he is too young to get married.
"But I am led to believe that there is no-one else involved and that it is a decision they have come to mutually."
Duncan Larcombe, the Sun's royal correspondent, told BBC Five Live: "The last person on earth who's going to be pressured by newspaper columnists and television chat shows to get married is Prince William."
The prince asked the paparazzi to stop harassing his girlfriend
The Sun said the pair had reached an "amicable agreement" to part.
Earlier this month, Miss Middleton settled a harassment complaint against the Daily Mirror over a close-up photograph published by the paper.
At the time of her 25th birthday in January, amid mounting speculation about a possible engagement, a media scrum formed outside Miss Middleton's home.
And police officers were called to a London nightclub that month to stop photographers getting too close as the couple left the venue.
Prince William, who is second in line to the throne, appealed for the paparazzi to stop harassing Miss Middleton.
Her lawyers contacted newspaper editors in 2005 to ask the press to leave her and her family alone.
They said that photographers had followed her almost every day and night since she had left university.
William, 24, is an army officer in the Household Cavalry's Blues and Royals.
Miss Middleton attended the prince's graduation from Sandhurst Military Academy in December.
It was the first time she had attended such a high-profile event, at which the Queen and other senior royals were present.
Miss Middleton left her family home in Berkshire on Saturday morning without speaking to the assembled press pack.
She works as an accessories buyer for a fashion company.