President of the European Parliament, Martin Schulz, adresses his guest at the "Europe Calling" conference | John Macdougall/AFP via Getty Images Martin Schulz steps in to stop a speaker on transparency Parliament president intervenes to prevent testimony from former staffer who is suing the institution.

A European Parliament committee chairwoman said Thursday the assembly's president, Martin Schulz, had intervened to prevent a former parliamentary staffer from speaking in a hearing on transparency.

Cecilia Wikström said the Parliament’s petitions committee had invited the former head of the Parliament’s civil liberties secretariat, Emilio De Capitani, to speak Tuesday at a debate entitled “Transparency and Freedom of Information within EU Institutions.”

But in a letter sent to Wikström before the hearing, Schulz said he would not authorize the hearing with De Capitani, citing an ongoing legal dispute.

“I would like to express my astonishment at the proposal of your committee,” Schulz wrote. “I regret to inform you that the hearing cannot be authorized given the possible prejudice of the dignity of the Parliament.”

Schulz told Wikström to resubmit a "corrected" program for the hearing, which went ahead on Tuesday, without De Capitani's participation.

De Capitani, who now works as a visiting law professor at Queen Mary University of London, is suing the Parliament in the European Court of Justice, claiming a lack of transparency in informal negotiations between EU institutions during the lawmaking process. De Capitani was unavailable for comment.

“The President interfered in a way that is unprecedented,” Wikström said, calling Schulz's decision a "gag" on her panel. “Isn’t Emilio in his right, as an EU citizen, to litigate against anyone he wants?”

According to parliamentary rules for public hearings, the assembly's president must be given "all relevant information and, in particular the names and capacities of the guests."

But Wikström said "he has no right to formalize or cancel the hearing."

A European Parliament spokesperson declined to comment.