The couple met while studying together at the University of St Andrews in Fife in mid-2001 and began a relationship around Christmas 2003 amid intense media pressure.
"As far as Kate is concerned, William simply hasn't been paying her enough attention," a close friend told the Sun.
"She is stuck in London while he is living in an officer's mess in Dorset. Kate feels hugely frustrated that their relationship just seems to be going backwards at a rate of knots."
The source also added that the couple only meet once a week due to the 24-year-old prince's busy army schedule.
Miss Middleton has also felt the strain of media attention since she began dating the prince and the pressure has contributed to the break-up. She recently lodged a complaint with the Press Complaints Commission over alleged harassment, which was settled last month.
"The sheer amount of attention Kate was getting put an unbearable strain on William. He, more than anyone else, will not be pressured into marriage by a media scrum hungry for an announcement," the source added.
Clarence House has so far not responded to the news.
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