The chairman of European champions Porto was questioned on Friday as part of an investigation into alleged attempts to influence referees.

Jorge Nuno Pinto da Costa, the Porto chairman, arrived voluntarily at a courthouse in Gondomar to answer questions from police.

He was formally detained on arrival, questioned and released more than three hours later.

TSF radio said Pinto da Costa was released on condition of reporting periodically to police and remaining in the Porto area.

He will have to return to court on Tuesday, the day when they host England's Chelsea in a Champions League match.

A police or court spokesman was not immediately available to comment.

Four referees and an agent were detained on Thursday for questioning in the 'Golden Whistle' investigation into attempts to influence match officials.

Officers of the plainclothes Judiciary Police searched the Porto offices as well as a Pinto da Costa home on Thursday, Lusa news agency reported. The club chairman had been in Spain at the time.

Police launched the investigation into alleged soccer corruption in April, focusing on the Porto area.

At the time, Valentim Loureiro, chairman of the Portuguese Professional Soccer League and mayor of Gondomar, was named as a suspect in 23 cases.