The Parliament, the only directly elected institution in the European Union, was the subject of particularly tough criticism by Transparency International, which cited a lack of cooperation by the lawmakers while it gathered research from June 2013 to February 2014.

“We met delays, prevarications and long silences by the authorities at the Parliament, and we find that acutely worrying in light of the body’s growing powers,” Carl Dolan, the director of the European office of Transparency International in Brussels, said ahead of the report’s publication. “The attitude of the Parliament’s hierarchy shows that there is a tendency there to circle the wagons and take the attitude of, ‘You’re either for us or against us,’ ” he said.

A spokeswoman for the Parliament, Marjory van den Broeke, said that the institution was already the subject of rigorous scrutiny from other European bodies and that there was no reason to give more access to Transparency International than to other nongovernmental organizations. “Compared to other parliaments, it’s a very transparent parliament,” Ms. van den Broeke said.

Other institutions, including the European Commission, the union’s administrative arm, took a more helpful approach by granting interviews with senior administrators, Mr. Dolan said.

Political analysts, including Vivien Pertusot, the head of the Brussels office of the French Institute of International Relations, are predicting that nearly 20 percent of the 751 Parliament seats up for election in May will be won by parties that are intent on reining in the European Union’s power, like the National Front in France and the U.K. Independence Party in Britain.

“The E.U. is going to have to find a way to address intense criticism from these protest parties over issues like corruption,” said Mr. Pertusot, who had not yet seen the report by Transparency International.

Mr. Dolan said his group was publishing the report now because it wanted to participate in the debate about the future of the bloc since the financial crisis, which had shaken confidence in the European institutions.