Apple (AAPL - Get Report)  faces a potential hefty tax bill following a European Commission decision expected on Tuesday.

The case concerns tax breaks Ireland granted to the iPhone maker, which the EC in a preliminary ruling has said amounts to an illegal subsidy. Apple may be forced to pay billions of euros in back taxes.

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EU Competition Commissioner Margrethe Vestager is due to hold a press conference around noon local time to announce the decision.

Washington has raised concerns over the EC's approach, with the U.S. Treasury Department last week accusing the EC of anti-U.S. bias and warning that the EC's probes against Apple and other U.S. companies undermine multilateral progress made towards fighting tax avoidance.

Apple shares slid 0.4% in premarket trading to $106.82, after closing 0.11% lower on Monday.