WARSAW, Poland - Lawmakers in Poland questioned EU Council president Donald Tusk on Monday for an investigation of tax evasion by private businesses that took place when he was the country’s prime minister.

A parliamentary panel that had subpoenaed Tusk quizzed him about his 2007-2014 government’s failure to curb illegal business practices dealing with value-added taxes on goods and services. The current government estimates the unlawful practices cost the state budget billions of zlotys (hundreds of millions of U.S. dollars) in lost revenue.

Tusk said the Law and Justice party, which now governs Poland, opposed many of the steps his government proposed to curb abuse of the VAT system, adding that the problem of tax evasion by private businesses has continued since he left office.

He accused the panel of having a political bias that would lead lawmakers to preconceived conclusions about his allegedly poor performance as prime minister. Tusk said he appeared out of respect for the Polish state.

Government critics alleged the questioning was part of the right-wing ruling party’s efforts to discredit Tusk and his circle, the leading rivals of Law and Justice. Party members comprise a majority on the tax evasion panel.

The ruling party lawmaker leading the panel, Marcin Horala, said before Tusk’s formal appearance started that there was “supposition” he had insufficient control as prime minister over law enforcement agencies or failed to take proper action.

The panel plans to question other officials. It is not clear when it will finish the probe or make its findings public. If serious shortcomings are revealed, the conclusions could lead to a criminal investigation and charges.

Tusk’s term as European Council president expires in November. He is seen as a potential candidate in Poland’s presidential election next year.

Tusk told reporters after his appearance that the panel’s main purpose was to “strengthen the ruling party” and undermine the opposition ahead of Poland’s parliamentary election in the fall.

Another commission previously questioned Tusk about a 2010 plane crash that killed President Lech Kaczynski, the twin of powerful Law and Justice leader Jaroslaw Kaczynski.