European Commission President Romano Prodi is under fire for allegedly interfering in Italian politics.
Critics say Prodi should stick to European business
Critics in Italy and elsewhere in Europe have attacked Mr Prodi for urging the Italian Left to join forces in next year's European elections.
A prominent European Parliament member has harshly criticised the move.
He said it clashed with Mr Prodi's role as commission president - especially at a crucial time for the future of EU reforms.
"I find Prodi's behaviour unacceptable," said Hans-Gert Poettering, leader of parliament's largest group, the conservative European People's Party.
"This is an improper conduct for someone who holds an office which should guarantee neutrality for everybody," Mr Poettering said.
He also slammed Mr Prodi for interfering with Italian domestic politics rather than tackling EU matters, and warned: "Prodi should stop, otherwise there will be serious consequences".
The attack was echoed by Italy's governing coalition, which sees Mr Prodi as the only politician who could beat Prime Minister Silvio Berlusconi at Italy's general elections scheduled to take place in 2006.
"Poettering is right: Prodi's behaviour is irresponsible," Economy Minister Giulio Tremonti said.
Deputy Prime Minister Gianfranco Fini also disapproved of the move.
"Prodi has taken to the field as a member of the centre-left, forgetting that as long as he is the president of the commission, he has the duty to represent all Italians and all Europeans, without losing sight of his role," he said.
The row began when Mr Prodi circulated a 50-page document called "Europe: the dream, the choices", in which he urged the Olive Tree opposition bloc to present a joint candidates' list.
Mr Prodi has played down the controversy.
He said his manifesto was his "reflections on Europe", and that he had no intention of interfering with Italian domestic issues.
"Nobody can even vaguely think that I do not dedicate 100% of my activity to Europe. We are working very hard, so this problem does not exist," he said in Brussels.
Back in Italy, Senate Deputy Speaker Roberto Calderoli from the Northern League later urged Mr Prodi to resign.
Mr Prodi once beat Mr Berlusconi at the polls in 1996.
Many in Italy think that he will make his comeback to Italian politics when his European mandate expires next year.