The European Commission is investigating Belgium's tax system -as it believes that it could be allowing multinationals to significantly reduce their tax bills.

The European Union in concerned that the country's "excess profit" tax system could constitute state aid and break EU rules. It allows companies to avoid paying tax on between 50 and 90 percent of their profits.

A statement from Margrethe Vestager, the commissioner in charge of competition policy, said that this system appears to grant substantial tax reductions only to certain multinational companies - allowing them to "pay no taxes on large parts of their profits, without any valid justification."

It continues to outline how it suspects that the country is exploiting rules aimed at avoiding the double-taxing of multinationals to reduce the tax liabilities of certain companies. The statement says "this would obviously be unfair and a distortion of competition."

Tax avoidance is not illegal - but the EU has strict rules regarding the tax incentives that member states can offer to companies.

The EU is currently investigating Amazon and Fiat's taxes in Luxembourg, Apple's payments in Ireland, and Starbucks' taxes in the Netherlands.