The European Union said it still has "serious concerns" over Poland's adoption of a law that is purported to tackle Russian influence in its domestic politics but is widely seen as a way for the country's ruling party to target the opposition.

Poland - a neighbor of Ukraine and one of its strongest supporters amid the conflict with Russia - set up a committee in late May with the intended goal of investigating citizens who have allegedly succumbed to Russian influence.

Despite criticism that it could serve as a tool to persecute political rivals ahead of a national election due this autumn, Poland's President Andrzej Duda - an ally of the ruling Law and Justice (PiS) party - signed the law into force on Wednesday.

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Brussels had launched legal action against Warsaw over the bill, which if it had been left unamended, could have been used to ban those found 'guilty' of holding sensitive public positions for up to 10 years. Under pressure, Polish President Andrzej Duda signed an amended bill into law on Wednesday that removed the threat of a ban from public office.

Instead, the panel will issue a statement that the person has succumbed to Russian influence and cannot be guaranteed to work properly in the public interest. Despite the last minute change, critics argue the measure, introduced just months before parliamentary elections, will be used to specifically target opposition leader and former European Council President Donald Tusk. 

His party even dubbed it the "Lex Tusk," or Tusk Law, because of what they said was its true aim. A spokeswoman of the European Commission said the bill risked putting Poland at odds with EU laws when the panel starts operating.

Anitta Hipper said the EU executive had already told Warsaw that "even if the amendments to the law aim to bring improvements, the law as amended continues to raise serious concerns."

There were still issues regarding "compliance with the EU law once the committee becomes operational and launches checks," she added.

Poland's governing conservatives have faced off with Brussels since coming to power in 2015 over claims Warsaw fails to fully uphold EU laws, particularly with regards to the judiciary. Tusk, a former prime minister of Poland and European Council chief, is posing a major challenge to the ruling PiS party ahead of elections later this year.

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