The prince's move to an army camp in Dorset, with Middleton continuing her life in London, is also said to have added to the strain.

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 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."

Earlier this month, 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 her home.