"Nicole was originally planning to use prosthetics but was told by the director, Stephen Daldry, that it would be more realistic if she gained the weight."

The Oscar-winner was snapped this month in a tightfitting white dress in New York sporting what looked like a baby bulge and a bigger cleavage.

"Nicole usually has a flat stomach so when she was pictured looking more curvy, everyone thought she was pregnant," said the source. "In fact, she has been putting on weight and has had to buy bigger clothes."

The actress follows in the steps of Zellweger, who piled on 30lb to play Bridget Jones in two movies by eating a diet of pizza, peanut-butter sandwiches and fast food and drinking Guinness.

She ballooned from a dress size six to 14 before dieting back to her normal weight.