The couple had been under much public scrutiny since they met in college in 2001, the BBC said. Additional strain was put on the relationship when William, 24, moved to an Army camp in Dorset while Middleton continued her life in London, the BBC reported Saturday.
The couple was popular with the British public and there had been speculation about a royal engagement.
Middleton settled a harassment complaint this month against the London Daily Mirror over a close-up photograph published by the newspaper. William, too, had called for the paparazzi to stop harassing Middleton.
Middleton attended William's graduation from Sandhurst last year and photographers had followed her daily since then.