The U.S. Treasury said it opposes plans by 11 European Union countries to impose a small tax on trades in shares, bonds and derivatives.

The European Commission, the EU's executive arm, on Thursday proposed a new financial-transaction tax it says could raise as much as €35 billion ($47 billion) a year for the 11 participating states, which include Germany, France, Italy and Spain, according to documents to be published Thursday and seen by The Wall Street Journal.

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