BRUSSELS (Reuters) -The European Commission will take the first step in a legal challenge against Poland over a Polish law on undue Russian influence that critics says could result in banning opposition politicians from public office.

A Commission spokesperson said on Wednesday that a letter of formal notice would be sent to Warsaw on Thursday.

Poland's minister for EU affairs, Szymon Szynkowski vel Sek, responded to the move by saying in a tweet that the goal of the law was to limit Russian influence in Poland and Europe and that democratic nations and institutions should unite around it.

"We will calmly provide the legal and factual arguments in this case after getting acquainted with the doubts of the European Commission," he wrote.

The U.S. government has also voiced concerns over the law, which would allow a panel to investigate whether opposition parties allowed Poland to be unduly influenced by Russia.

The liberal PO (Civic Platform), in government from 2007 to 2015, says the law is designed to destroy support for its leader and former prime minister Donald Tusk ahead of a national election scheduled for October or November.

Poland's President Andrzej Duda said on Friday he would propose amendments to the law.

(Reporting by Marine Strauss, Philip Blenkinsop in Brussels and Alan Charlish in WarsawEditing by Gareth Jones)