"We've always said that we take their opinion very seriously but it's not an ECI as the legislation is not yet in place," Michael Mann, the commission's administration spokesman, told this website on Tuesday (30 November).

"Strictly speaking, they would have to do it all over again," he added. "The Greenpeace view that the petition counts as the Lisbon Treaty is in place doesn't stand up to legal scrutiny."

A European Parliament official concurred. "We may end up going to court on this," the contact said.

Greenpeace argues the Lisbon Treaty's coming into force on 1 December 2009 makes their petition over the controversial GMO issue legitimate. "We disagree with the commission and we have a legal opinion to back this up," said the group's communications officer, Jack Hunter. "It would be political madness to ignore the petition, especially after such a low turnout in the last European elections," he added.

The dispute comes as MEPs in the European Parliament's constitutional affairs committee made a fresh push on Tuesday to alter the draft legislation setting up the citizens' initiative, unanimously approving a report on the subject.

Changes include an earlier check on the admissibility of an ECI, rather than after 300,000 votes have been collected as originally proposed by the commission in spring. If MEPs get their way, a check will be carried out as soon as a particular initiative is registered with the commission.

Member states themselves have called for the threshold to be lowered to 100,000 signatures.

Once an initiative is registered, MEPs say signatories on a petition should come from at least one fifth of EU member states, a lower threshold than the one third proposed by the commission and backed by member states.

A meeting between the three institutions on Tuesday evening is likely to settle on a one fourth compromise, say EU sources.

MEPs also sought to simplify the signing of petition initiatives by deleting a requirement to give an ID card number, arguing that a name, address, nationality and the date and place of birth should be enough. A number of member states have called for ID numbers to be included.

The organisers of any initiative that manages to collect the required 1 million votes within the 12-month time period should then be invited to a public hearing by the commission and the parliament, said the MEPs.

"We decided that it's not enough for the commission to just send a letter saying 'Yes' or 'No'," said Green MEP Gerald Hafner, one of the report's authors.

Asked whether the initiative could be hijacked by extremist groups, centre-right MEP Alain Lamassoure said: "The time is always right to introduce more democracy into the system." The process will be positive, even is used by political parties, lobbyists, euro-sceptics, he argued, so long as it is transparent.

If the two sides manage to reach an agreement, ambassadors from member states will give their approval to the citizens' initiative on 9 December, allowing the fully plenary of MEPs to vote on 16 December. A few months will then be required for the procedure to be fully put in place.