Inquiries were opened following press reports of a vast system of carbon credit fraud involving German oil companies taking part in climate projects, some of which never actually existed.

In other cases, the announced CO2 reductions did not correspond to the actual savings.

"Serious legal and technical incoherencies" were found in the first seven cases examined by the Federal Environmental Agency (UBA), while an additional eight case had broken regulations on when it could start operating.

The refused credits represent emission reductions equal to the equivalent of 215,000 tonnes of CO2 that the groups were planning to reflect in their climate balance sheets.

The names of the companies were not revealed.

The alleged malpractices concern a scheme -- suspended since July -- allowing German oil companies to comply with legally prescribed climate targets through environmental projects in China.

Out of a total of 69 projects, 40 must still be investigated by the UBA, with 21 of them strongly suspected of wrongdoing.

Berlin prosecutors in July raided the offices of environmental auditing firms that may have been complicit in the alleged fraud.

By embellishing their climate balance sheet, the companies avoided fines adding up to as much as 4.5 billion euros ($5 billion), according to estimates by the German biogas federation.