The One Day actress will play Fantine in the upcoming adaptation from Tom Hooper. Her character suffers from poverty and abuse in the musical, also starring Hugh Jackman.
She will allegedly eat just two apples a day in order to lose 16lb (1.14 st) in less than 20 days to shoot a tragic scene for the part, reports The Mirror.
"Anne is playing a destitute factory worker-come-lady of the night," an insider said.
"Unfortunately, she only has 15 to 20 days to lose as much weight as possible – up to a stone and a half – because she's been filming the scenes showing her fit and healthy, almost slightly chubby.
"It's not ideal but, as with any high-budget movie, there are strict time constraints."
On how she will lose the weight in such a short timeframe, the source added: "Producers have assigned her a personal trainer and she is literally eating nothing more than a couple of apples a day, and some protein – mainly in the form of a shake.
"Anne knows the risk of such an extreme diet and will undergo weekly medicals to make sure everything is still functioning as it should do."
Russell Crowe, Amanda Seyfried, Helena Bonham Carter and Sacha Baron Cohen are all set to star in Les Misérables, which has started filming in Portsmouth.
Leading star Jackman recently posted the first official photo of himself as Jean Valjean on Twitter.
Les Misérables will be released to cinemas on December 14.