The 26-year-old actor's natural voice had started to fade after playing a string of American characters, including Twilight's Edward Cullen.
'They hired an accent coach as Rob's spent a lot of time in LA,' his Bel Ami co-star Christina Ricci, 32, revealed to US chat show host Jimmy Kimmel.
'He made fun of me as I was trying to be British and sounded like a Hollywood valley girl.'
But it seems Barnes-born Rob isn't that bothered about keeping his London tones.
He recently admitted that he prefers to play characters with different nationalities to his own.
'I feel very naked with my British accent,' Robert told The Sun. 'I feel like I'm not really acting.'