The European Commission has opened an in-depth investigation into a Belgian tax provision allowing companies to reduce their tax bills.
The probe is part of a broader inquiry into tax deals with multinationals across the European Union.
The deductions granted through Belgium's "excess profit" tax system usually amount to more than 50% of profits and can sometimes reach 90%.
Excess profits are calculated as being advantages, such as economies of scale or intra-group synergies, that result from being a multinational group.
The Commission said the deductions significantly overestimate the benefits of being in a multinational group.
"The Belgian 'excess profit' tax system appears to grant substantial tax reductions only to certain multinational companies that would not be available to stand-alone companies," Margrethe Vestager, commissioner in charge of competition policy, said.
She added that, if the Commission's concerns were confirmed, the scheme would be a serious distortion of competition unduly benefiting a selected number of multinationals.
The Commission is also investigating the tax arrangements of Amazon and Italian carmaker Fiat in Luxembourg, Apple in Ireland and Starbucks in the Netherlands.
In December, the Commission asked all 28 member countries for details of tax deals made with companies between 2010 and 2013.
Tax avoidance, while not illegal, has in recent years galvanised authorities into taking action to try to ensure that multinational companies pay a fair share of their profit in tax.
