"It's increasing big, big, big time - it's the third largest shadow economy after drugs and small arms," the 48-year-old actress-screenwriter told reporters.
A United Nations human trafficking forum begins Wednesday.
Thompson is the chair of the Helen Bamber Foundation, a Britain-based group that helps rebuild the lives of victims of cruelty.
She warned it was much easier to buy and sell people - especially women - who do not have birth certificates.
"In many, many countries in the world, girls simply aren't registered and they get lost and they get sold and they get used - and I know that because I've spoken to lots of them in lots of different countries," Thompson said.
She said human trafficking had increased exponentially over the past decade and that traffickers were fantastically sophisticated.
Sometimes traffickers are neighbors, uncles, brothers - even fiances and often other women who have families and daughters of their own, Thompson said.
"All the stories are different - but mostly they involve trust and the breaking of someone's trust," said Thompson, who won a screenwriting Oscar for "Sense and Sensibility" and a best actress Oscar for "Howards End."
The U.N. gathering expects to draw more than a thousand experts, legislators, law enforcement teams, business leaders, non-governmental organizations and trafficking victims from more than 100 countries.
