The EU has threatened to take action against Poland, after MPs approved a new commission which could bar people from public office for links to Russia.
Approved by Poland's parliament last week, the commission will probe alleged Russian interference between 2007-2022.
But critics say the panel, which will be dominated by government MPs, is designed to attack opposition leader and ex-prime minister Donald Tusk.
EU Justice Commissioner Didier Reynders said the panel was a "special concern".
The commission will be empowered to issue 10-year bans from managing public funds - in effect, barring them from national office - to anyone found guilty of acting under "Russian influence".
In particular, it will investigate gas deals signed with Russia, which the government says left the country overly reliant on Moscow.
The 10-member commission is expected to be dominated by MPs from the governing Law and Justice Party (PiS) and could deliver its first report as soon as September, reports say.
Mr Reynders told reporters in Brussels that the EU Commission "will analyse the legislation and will not hesitate to take measures if it is needed".
"It is impossible to agree on such a system without a real access to justice, to an independent judge against an administrative decision," he added.
MPs from the opposition Civic Platform Party fear the investigation - which will cover its last term in office between 2007 and 2015 - will seek to damage support for Mr Tusk, who then served as prime minister.
Mr Tusk is now the party's chairman and leader, though he is not an MP, and is expected to challenge Prime Minister Mateusz Morawiecki in elections later this year.
Civic Platform's parliamentary leader, Krzysztof Brejza, has called the new commission a "Soviet-style idea" and accused the government of "organising a witch hunt against Donald Tusk and eliminating him" before the poll.
But Mr Morawiecki has defended the law and accused Mr Tusk of having something to hide.
"There is nothing to be afraid of," Mr Morawiecki said, adding: "Why is this esteemed opposition of ours, especially Mr Tusk, so afraid of a commission to verify Russian influence?"
On Monday, the US Ambassador to Poland, Mark Brzezinski, said he feared the commission could "reduce voters' ability to vote for those they want to vote for".
But Poland's foreign ministry insisted on Tuesday that "any party subject to a Committee's decision will have the right to appeal" and said the panel "will not limit voters' ability to vote for their candidates in elections".
Warsaw is already locked in a long-running battle with the EU over reforms to the judicial system, which saw the bloc suspend billions of euros in aid to Poland in January.
PiS has also been accused of rolling back other civic freedoms, including freedom of the press, and opposition MPs say Poland risks becoming an authoritarian state if the government is re-elected.
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