The 25-year-old has told work colleagues the break-up was "because of his daddy".
In an interview with Britain's Daily Mirror, Middleton's colleagues told how they watched as the brunette lost her cool, unable to hide her bitterness any longer.
"She had about 10 people asking if she was all right, and I think she just lost her rag," colleague Philip Higgs said.
Higgs said Middleton had been in her office at the fashion chain Jigsaw in London, when she made the comment to workmates.
"Kate said: 'It's because of his daddy.' Everyone raised their eyebrows, but didn't want to push her."
Since the split almost three weeks ago, speculation has been rife over what caused Prince William to end his four-year romance. The Queen has been named as one possible source of interference.
But according to the Daily Mirror, Middleton has pointed the finger squarely at Prince Charles.