Poland on Wednesday adopted an amended version of a panel to probe "Russian influence", after an avalanche of EU and US criticism of the move, widely seen as targeting the opposition.

Poland -- a neighbour and staunch ally of Ukraine, which is battling Russia's invasion -- in late May set up a committee with the stated goal of investigating citizens who may have succumbed to Russian influence.

Under the law, those found guilty could have found themselves banned for 10 years from public positions relating to public finances and classified information.

Critics argued the measure, introduced just months before parliamentary elections, would be used to target opposition leader Donal Tusk.

His party has even gone so far as to dub it the "Lex Tusk", or Tusk Law, due to its suspected aim.

Following the law's adoption, the European Union launched legal action against Poland, while the United States expressed concern that the law "could be used to block the candidacy of opposition politicians without due process."

Faced with the criticism, Polish President Anrzej Duda proposed amendments to do away with the penalties and on Wednesday, signed the amended bill into law.

Under the revised legislation, a person whom the committee finds to be under Russian influence would not be banned from holding public positions. 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.

Additionally, the nine committee members will now be experts, instead of lawmakers and senators.

Committee decisions could be appealed before an appeals court.

Critics say the changes do not go far enough.

Poland's governing conservatives have been at loggerheads 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 president, is posing a major challenge to the ruling Law and Justice party (PiS) ahead of elections later this year.