But Verhofstadt, who leads the Parliament's Liberal group, warned that such backroom deals were now over.
The European Parliament, he said, would have full co-decision powers on legislative proposals that will come out later in the year to flesh out the EU's new economic governance.
His warnings were echoed by other political groups in Parliament, including the centre-right European People's Party (EPP), which commands the largest number of seats in the Strasbourg assembly.
Iñigo Mendez De Vigo, a Spanish MEP in charge of institutional issues at the EPP, said he welcomed the Task Force's proposals. But he added that "they should take into account that the European Parliament is now co-legislator and will play its full part in defining the reforms to come".
"I regret that the French-German proposal does not even mention the European Commission, which also has a say on this issue," De Vigo said, adding the Parliament should also be more involved.
The Greens, the fourth largest group in Parliament, also backed the Liberals and the EPP, in a move which could herald a long battle with member states over the economic reform plans.
The Parliament "will be a co-legislator on four of the six legislative proposals" on economic governance, said Belgian MEP Philippe Lamberts, saying his group was "in favour of a more ambitious and broader economic framework than the Commission and Council".
Verhofstadt said he hoped this new battle would not take nine months, referring to the time it took to pass a recent package of financial supervision laws through the assembly.