GENEVA (Reuters) - Why is that tomato so cheap?

Campaigners are asking shoppers to find out where the food and goods they buy come from to avoid unwittingly supporting a modern form of slave labor with their purchases.

The "Buy Responsibly" television advertising campaign, designed by M&C Saatchi and which will run in Europe, features exploited workers trapped under an upside-down shopping trolley.

"They're just here to pick the tomatoes we buy everyday," the 30-second spot says of the encaged people.

Trafficked and trapped migrants provide cheap labor in construction, agriculture, fishing, textiles and other sectors whose products end up on rich-country shop shelves, according to the International Organization for Migration.

"They are picking agricultural produce or producing consumable items that we all go and buy," said Richard Danziger, head of IOM's global counter-trafficking program.

"We are not asking people to boycott a particular brand or a supermarket or chain store. We are simply asking people to find out what lies behind the product they buy, for people to buy responsibly," he told a briefing in Geneva, where IOM is based.

As many as 12.3 million people worldwide are caught up in forced or bonded labor and sexual servitude, according to IOM estimates. To find out about how consumer habits can support -- or fight -- human trafficking, shoppers can visit a website www.buyresponsibly.org.

(Reporting by Stephanie Nebehay; Editing by Laura MacInnis)