The 19-year-old student pictured in a Sunday newspaper in a nightclub with Prince William said the heir to the throne was the "perfect gentleman".
Pictures in yesterday's Sunday Mirror showed the prince with his arm round Lisa Agar in the Elements night club in Bournemouth.
The paper claimed he was seen "downing pints of Stella Artois and shots of sambuca" and "gyrated hip to hip" with Ms Agar.
His long-term girlfriend Kate Middleton, the red top suggested, was likely to be furious.
But speaking to her local daily, Ms Agar, an engineering student, said the prince had been on his best behaviour.
"We had a couple of dances and he was great fun, but he was perfectly well behaved, absolutely impeccable," she told the Daily Echo.
"Anyone who says any different is not telling the truth."
William was on a night out with colleagues from the Household Cavalry based at nearby Bovington Camp.
Ms Agar described the 24-year-old royal as "really down to earth", rubbishing claims that he had begged her to come back to his barracks.
"He was relaxed and friendly and we chatted about music and things like that," she added.
William's younger brother Harry is also in the papers today, after photographers snapped him leaving a central London nightclub.
The prince fell over on the pavement, but friends later claimed he was not drunk and had simply lost his footing.