Scientists from Brazil and the US say new research suggests deforestation of the Brazilian Amazon has been underestimated by at least 60%.
The team has completed a study using a more advanced technique of satellite imagery that can pick up more types of logging activity.
These include selective logging, where loggers pick out trees of value but leave the surrounding forest intact.
Brazil's government welcomed the report but said the figures were exaggerated.
Nasa's help
Deforestation in the Amazon is on such a massive scale that the only way of measuring it is by using satellites.
The trouble has been that while traditional aerial images can show areas that have been completely destroyed, they do not reveal selective logging of valuable trees such as mahogany.
With input from the Nasa space agency, the joint US and Brazilian team used an ultra-high-resolution technique to examine just how much selective logging was going on.
The report was published in the US journal Science.
The researchers concluded that the area of rainforest destroyed between 1999 and 2002 was thousands of square kilometres bigger than previously thought.
They also found that about 25% more carbon had been released into the atmosphere than estimated - possibly enough to affect climate change.
Brazilian officials praised the scientists for highlighting the issue of selective logging, but said the new figures were hard to believe.
The businessmen involved in the practice claim picking out individual trees is more environmentally friendly than the blanket clearance of huge areas.
But environmental campaigners say that to reach the prized trees, roads have to be built and heavy equipment brought in.
This, they say, can be of no benefit to the Amazon.
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