The European Commission warned Microsoft on Wednesday that it risked an investigation into its new Windows 8 software if the company did not meet commitments on web browser choice.

Microsoft had not provided clients a web browser choice on Windows 7 in 2011-12, as agreed, the Commission said, and it had now raised similar concerns over the US giant's next generation Windows 8 due to launch later this week.

European Competition Commissioner Joaquin Almunia said he had been in contact with Microsoft at the "highest levels" to make the point that they should not repeat the same mistake in the new software package.

It was essential that Microsoft give customers a clear and simple web browser choice "if they don't want to take the risk of a new investigation," he told a press conference.

If companies "enter into commitments they must do what they committed to do otherwise they must face the consequences," Almunia added.

Microsoft will launch Windows 8 and its Surface tablet computer, designed to compete with Apple's popular iPad, in the United States and China on Friday.

Windows remains the dominant platform for personal computers but it has lost ground to Apple and Google in newer devices which use rival operating systems so there is a huge amount riding on the success of Windows 8.

Almunia said the Commission had also looked at Windows RT, the software designed to run the new Surface tablet, but had found no cause for concern.

In response, Microsoft said that after discussions with the Commission, it was making some adjustments to Windows 8.

"We are changing some aspects of the way the Browser Choice Screen works on Windows 8 and will have those changes implemented when Windows 8 launches later this week," it said.

Microsoft also apologised for what it said was a "technical error" on Windows 7, as it did in July when the EU launched its probe into why 28 million users were unable to choose between the company's default Internet Explorer and other browsers.

On Wednesday, the Commission said it had sent a preliminary statement of objections to Microsoft, charging it with failing to offer users a web browser choice on Windows 7 between February 2011 and July 2012.

As a result, "millions of Windows users in the EU may not have seen the choice screen. Microsoft has acknowledged that the choice screen was not displayed during that period," a commission statement said.

The statement of objections is a formal step which allows the concerned party to reply, with a final decision only coming after they have made their defence.

Asked how much the fine might be, Almunia said it was too early to say.

"It is a serious infringement," he said, adding that the investigation sent a strong message to other companies that the EU was serious about dealing with competition issues.

Under EU law, a company found to have breached commitments given to resolve competition cases can face a fine of up to 10 percent of its annual sales.

In its last results for the three months to September, Microsoft reported sales of $16 billion.

The EU fined Microsoft 899 million euros ($1.2 billion) in 2008 for failing to comply with an order to share product information with rivals so that their software could work with Windows.