A marketing stunt launched Monday in Brazil to track some buyers of Omo washing powder through a GPS chip hidden in the box for a prize giveaway risked backfiring with observers raising privacy concerns.

"Is your detergent stalking you?" the influential US trade magazine Advertising Age asked in an article printed on its website.

The technology website CNET noted the promotion would make some shoppers "a marked man or woman."

Specialist publications also questioned the reaction of consumers in a high-crime country such as Brazil where strangers knocking at the door to award giveaways could be viewed as potential robbers.

Under the promotion, Unilever, the Anglo-Dutch consumer products group that makes Omo, this week started planted the GPS tracking chips in 50 of the detergent boxes.

The idea was to track the shopper to their home, where an Omo team would unexpectedly turn up to announce they had won a camcorder and an adventure weekend.

The use of the GPS chips meant the teams "may get to your house as soon as you do," Fernando Figueiredo, the head of Unilever's Brazilian promotions agency Bullet behind the stunt, was quoted as saying.

Teams stood by in 35 cities to surreptitiously follow the unsuspecting shoppers, he said.

Sao Paulo-based Bullet said on its own website that the stunt would run to September 19 and involved GPS chips with signals that could only be read by its teams' equipment.

The aim was to give consumers "a totally new and different experience," it said.

Advertising Age said that if a wary shopper refused to let the Omo team in, the teams could make the GPS device buzz. If that didn't work, a note inside the detergent box would explain the promotion and give a phone number to call.

Omo accounts for nearly half the clothing detergent sales in Brazil.