BEIJING (Reuters) - Shanghai has banned taxi drivers from yawning and snacking on the job in a new drive to smarten up the city's cabs, just months after suggesting portable spittoons for cabbies, a Chinese newspaper reported on Friday.

But the Beijing News did not say what punishment the drivers would face for a careless yawn or a noisy nibble on a snack, and quoted a taxi company boss saying the rule would be hard to enforce.

Chinese taxi drivers, for instance, are already banned from smoking behind the wheel, but a casual observer would be hard pressed to tell.

Shanghai residents told the paper they thought the no-yawning rule was harsh on drivers who work shifts that can last up to 24 hours for relatively little pay.

Shanghai authorities said in January they planned to distribute 45,000 "spit sacks" to cabbies to curb the common habit of rolling down the cab window and expectorating on to the street.