BRUSSELS — The European Commission on Tuesday opened an investigation against Microsoft for failing to take sufficient measures to open up the market for Internet browsers, a step representing a resumption of hostilities between European regulators and the US software giant.

Joaquin Almunia, the European antitrust chief, said at a news conference that Microsoft had not complied with commitments made in 2009, which he called a serious breach of EU antitrust rules.

‘‘If the infringements are confirmed, there will be sanctions,’’ said Almunia, adding that he would use ‘‘legal instruments with all my capacity to deter and to punish.’’

The sanctions could be particularly severe because it was the first time a company had defied a settlement decision offered by the commission, Almunia said.

The renewal of hostilities with Microsoft represents a major setback for the commission’s efforts to find solutions to antitrust problems, particularly in the fast-moving technology field, so that cases no longer drag on for years.

The commission has fined Microsoft about $2 billion over the past decade for breaches of EU law linked to an abuse of dominance over the computer desktop through the Windows operating system.

Almunia has been considering a settlement offer from Google.