Microsoft is facing another hefty fine after the EU announced it was reopening its antitrust investigation into the firm’s Internet Explorer (IE) browser.

The company is supposed to offer users a choice of which browser to use, a process introduced in 2009 after it was judged that offering only IE was against fair competition rules.

However it appears that an update to the Windows 7 operating system removed the browser choice screen. EU competition commission Joaquin Almunia said the option may have been missing since February this year and that up to 28 million people may have missed out on seeing it.

Almunia said if the EU does consider this to be an infringement on its earlier ruling it will look to punish Microsoft. The EU can fine companies up to 10% of their annual revenue if they break previous rulings. Microsoft’s revenue for FY 2011 came in at $69.94bn, meaning the company is potentially facing a fine of around $7bn.

In a statement Microsoft said a technical error was to blame. "Due to a technical error, we missed delivering the BCS software to PCs that came with the service pack 1 update to Windows 7. While we have taken immediate steps to remedy this problem, we deeply regret that this error occurred and we apologise for it."

The statement added that users running the original version of Windows 7 as well as those on XP and Vista did see the screen.

Since the browser choice screen was introduced, Internet Explorer has steadily lost market share. From a dominant market share of around 70% in 2009, IE now claims around 30% of the market, almost level with Google’s Chrome browser. Firefox is just behind on around 25%, well ahead of Safari on about 8% and Opera on just under 2%.