The FAO defines a forest as an area larger than 0.5 hectares where 10 percent of the ground is covered by tree canopy.
The Rainforest Foundation said this definition was far too loose. "Ten percent is just land with a few trees dotted around. They are exaggerating the area of forest," Counsell said.
The canopy of a tropical forest often covers almost 100 percent of the ground. Environmentalists say when this figure falls below 50 percent, the forest’s eco-system is wrecked.
But the FAO defended its methodology, saying it was almost impossible to gauge the degradation inside forests, and warned against excessive alarmism.
It said primary forests, which are areas undisturbed by humans, represented 36 percent of total global forests, with some 6 million hectares lost or modified each year.
"It is obviously very sad to lose this amount, but you should bear in mind that it represents just 0.4 percent of total primary forest," said survey co-ordinator, Mette Loyche Wilkie.
FAO said plantations accounted for less than 5 percent of all the world’s forest areas, while 11 percent of forests were official conservation areas — up 96 million hectares on 1990.