EUROPEAN PARLIAMENT president Hans-Gert Pottering has called on Libertas founder Declan Ganley to reveal where he got the funds to campaign against the Lisbon Treaty.
He has also asked the parliament's political groups to consider launching an inquiry to discover whether US agencies actively supported Libertas in the June 12th referendum.
"We require total transparency because the people who attack us always call upon us to show transparency. We cannot have double standards," said Mr Pottering, who added that Mr Ganley needed to reveal where and how much money he spent in the Irish campaign.
Mr Pottering was responding to a speech by Green Party president Daniel Cohn-Bendit at the opening of the parliament's monthly plenary session in Brussels. Mr Cohn-Bendit called on the EU council to undertake an investigation into Libertas funding to determine if claims of interference by US bodies such as the CIA or Pentagon were true.
"We are awaiting confirmation of reports in the media regarding funding of Libertas's campaign for a No vote to the Lisbon Treaty. If proved true, this would clearly show that there are forces in the US willing to pay people to destabilise a strong and autonomous Europe," he later explained in a press statement.
"If this can happen for the Lisbon Treaty vote, it raises grave concerns for interference in next year's European elections."
The leaders of the parliament's political groups are expected to discuss the issue in coming weeks and determine whether they should ask the council to investigate the Libertas funding or establish its own committee of inquiry staffed by MEPs.
Sinn Féin MEP Mary Lou McDonald also called on Libertas to explain its funding.
"I too favour transparency in funding. I also favour the respect for democratic outcomes and that this body should note that all of the campaigns that fought and won the argument for a No vote with the exception of one have published full and frank accounts," she said.
Fianna Fáil MEP Eoin Ryan said his party had been open and transparent about its funding, but Libertas had "told us one thing and now we discover it is very different".
Last week Mr Ganley disclosed that he gave a personal loan of €200,000 to help fund the Libertas campaign against the Lisbon Treaty. During the Lisbon campaign, the communications director for Libertas John McGuirk said it had a budget of €1.3 million.