Greenpeace says illicit Indonesian wood is being used in the renovation of the EU's head offices in Brussels.
Indonesia loses huge areas annually to logging
The wood for two EU buildings comes from companies known to trade "in timber from Indonesia's threatened rain forests", the environmental group said.
EU officials said they were assured only environmentally sound wood would be used - and if it were illicit, they would "demand an explanation".
The EU has backed the global campaign against illegal logging.
Nearly 90% of logging in Indonesia is believed to be illegal, Greenpeace says.
The European Commission is due to move back soon to the Berlaymont building.
An EU spokesman said the use of environmentally sound wood for Berlaymont and the nearby Economic and Social Committee building was "written into the contract".
But Greenpeace declared them "Forest Crime Scenes".
"Not only does the EU allow the import of illegal wood
into Europe, it is fuelling the trade in illegal and
destructive timber through its own building projects,"
Greenpeace Global Forest Campaign Co-ordinator Gavin Edwards said.
"Indonesia's rain forests should be home to orang-utans
and tigers, not EU bureaucrats in plush offices and
chambers," he added.
The European Commission said it shared Greenpeace's concern and would ask the contractor, Berlaymont 2000, to investigate.