WARSAW/BRUSSELS (Reuters) - Polish prosecutors have summoned European Council President Donald Tusk as a witness in a case against former secret service officials that relates to events when Tusk was Poland’s prime minister, Warsaw district prosecutor’s office said on Monday.

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The summons announcement came two days after Poland’s foreign minister said the government would start obstructing European Union business as the bloc angered Warsaw by reappointing Tusk as chairman of EU summit meetings.

The eurosceptic Law and Justice (PiS) party, in power since 2015, has brought prosecutors under direct government control.

Prosecutor Michal Dziekanski said Tusk had been summoned to testify in the investigation against former heads of the military counter-intelligence services (SKW), who are “charged with co-operating with intelligence services of another country without the required authorization of the prime minister.”

But Tusk’s spokesman said the European Council president would not be available for the March 15 summons because he had prior engagements in the European Parliament.

“He will be in Strasbourg for the European Parliament’s plenary session on Wednesday, March 15,” Tusk’s spokesman said.

The former leader of Poland’s largest opposition party Civic Platform (PO), Tusk is a long-standing rival of ruling PiS party head Jaroslaw Kaczynski.

Polish media has reported the investigation is focused on the co-operation of intelligence officials with Russia, including the exchange of information after the Polish presidential jet crashed in Russia in 2010.

The former head of the SKW General Janusz Nosek, who was in charge in 2010, said the case was politically-motivated as Tusk was fully aware of the co-operation of secret services with Russia and had authorized it.

“In my opinion the whole case is particularly highly politically-motivated,” he told local radio RMF FM. “It is aimed solely at discrediting the leadership of secret services during PO’s rule, and indirectly also at discrediting the prime minister at the time.”

Kaczynski, 67, who holds no government post but is seen as Poland’s main decision-maker, said on Monday that he had nothing to do with summoning Tusk.

“I can swear on any sanctity that I learned about it today,” he told a press conference. “This has nothing to do with me, because I think the question was that this is some sort of personal harassment.”

Kaczynski had previously said that Tusk should not be reappointed to chair EU summits because he may face charges in Poland relating to the Smolensk crash and a Ponzi scheme.

Kaczynski’s twin brother, President Lech Kaczynski, was killed in the Smolensk crash and his sibling has said he holds Tusk’s government at least partially responsible for the disaster, citing negligence.

The PiS government has already clashed with EU’s executive arm, the European Commission, over issues including the rule of law, migrants, climate policy and environmental protection.

The EU’s executive arm last year accused the Polish government of undermining democracy and rule of law after the party overhauled the constitutional court to make it more difficult for judges to block new legislation.