In an announcement Wednesday, the commission said it has decided to undertake the more thorough Phase II review after its preliminary review raised several issues.

"Steel is a crucial input for many goods we use in our everyday life, and industries dependent on steel employ over 30 million people in Europe. Those European industries need access to steel at competitive prices to compete in global markets. This is why we will carefully investigate the impact of ArcelorMittal's plans to buy ILVA on effective competition in steel markets," Commissioner Margrethe Vestager said.

ArcelorMittal, along with Italian re-roller Marcegaglia, in June reached a binding agreement to acquire ILVA from the Italian government for EUR1.8 billion. ArcelorMittal said it would cooperate with the investigation. 

"ArcelorMittal will continue to work closely and constructively with the European Commission to explain the dynamics of the steel industry, the rationale of the proposed acquisition and the benefits it will bring to industry, customers, the environment and the local economy," the company said.