Hollywood actress Maggie Gyllenhaal is hiding a secret smoking habit.

The Dark Knight star, who is a mother to two young daughters, insists she doesn't puff on cigarettes regularly, but often falls "off the wagon" and buys a pack.

She tells Britain's Stylist magazine, "I don't smoke cigarettes officially but now and then I fall off the wagon. I can't keep it up at home. I can't smoke as a mother - it's too much sneaking around... I'm buying cigarettes and I'm like, 'What am I doing? Pull it together!'"