Experts said a rise in diabetes and other diseases had increased demand for bought and sold organs, leading to a lucrative industry of trafficking.
Organised gangs are believed to be harvesting organs such as kidneys from vulnerable people for patients offering almost £130,000 ($200,000), the Guardian reported.
Many of the patients involved in the operations, which were estimated to number 10,000 a year - more than one an hour, go to China, India or Pakistan for the procedures.
Luc Noel, a WHO official, who monitors the black market for organ donations, said: "The illegal trade worldwide was falling back in about 2006-07 - there was a decrease in 'transplant tourism.'
"The trade may well be increasing again. There have been recent signs that that may well be the case.
"There is a growing need for transplants and big profits to be made. It's ever growing, it's a constant struggle. The stakes are so big, the profit that can be made so huge, that the temptation is out there."
Figures from WHO showed more than 100,000 organs had been legally and illegally transplanted in 95 member states in 2010.
Mr Noel, a doctor, believed one in 10 was harvested illegally. He also estimated three-quarters of illegal organ donations were kidneys to treat patients with diabetes, high blood pressure and heart problems.
Jim Feehally, a professor of renal medicine at University Hospitals of Leicester NHS trust, told the Guardian: "We know of countries in Asia, and also in eastern Europe, which provide a market so that people who need a kidney can go there and buy one.
"The people who gain are the rich transplant patients who can afford to buy a kidney, the doctors and hospital administrators, and the middlemen, the traffickers. It's absolutely wrong, morally wrong."