European Commission chief Jose Manuel Barroso has dismissed allegations of misconduct over a holiday on a Greek billionaire's yacht as "absurd".
Jose Manuel Barroso says the claims are unfair and unjustified
A group of MEPs said the trip revealed a possible conflict of interest.
But Mr Barroso condemned the censure motion presented in the European Parliament as "gutter politics".
The 77 MEPs say that a company owned by shipping magnate Spiros Latsis later won commission approval for a 10m-euro ($12.6m; £6.9m) state aid package.
Mr Barroso, who was not then Commission president, denies doing anything wrong.
He accuses his critics of trying to damage the EU's image ahead of key votes in France and the Netherlands on the bloc's first constitution.
"The true agenda of this sort of attack is to undermine the credibility of the Union's institutions and its work of integration, replacing the debate on ideas with the fabrication of alleged scandals," Mr Barroso said.
A vote on the censure motion is expected in June.
Correspondents say the motion has no chance of being passed, because it lacks the support of the parliament's largest political groups.
The leaders of the parliament four main groups, who together occupy 597 of the parliament's 732 seats, said the initiative was a publicity stunt.
The action was launched by the leader of Britain's Eurosceptic UK Independence Party (UKip), Nigel Farage, after Mr Barroso refused to answer a parliamentary question on the holiday.
He said he wanted to have more transparency in the dealings of the European Commission.
"By ignoring my simple request and by maintaining that these free holidays do not constitute a conflict of interest, you have put yourself in an unenviable position," Mr Farage said during the debate.
He said that a $20,000 (£10,900) Mediterranean cruise on a billionaire's yacht qualified as a gift.
Mr Barroso acknowledged spending time on board the yacht owned by his friend Mr Latsis, but argued it was his right to have holidays with friends and family.
He gave up his supervisory role on antitrust procedures, including those related to the shipping industry, after news of the yacht holiday emerged.
The motion censures the entire Commission, because there is no procedure for censuring the President of the Commission himself. It needs a two-thirds majority to pass.
The Commission has a code of conduct for its commissioners which includes a public register of gifts worth more than 150 euros (£103) and declarations of interest.