European Union antitrust regulators are seeking to ensure that an offer by U.S. software company Microsoft to give users the chance to choose rivals' Web browsers was workable for consumers and its rivals.

Microsoft proposed to the European Commission in July to install a ballot screen that would allow users to set up rival browsers on its latest Windows operating system, a move which could end a decade-long dispute with the EU executive.

The Commission, competition watchdog for the 27-country European Union, has to date slapped a total 1.68 billion euros ($2.47 billion) in fines on Microsoft for infringing EU antitrust rules.

A confidential Commission questionnaire obtained by Reuters showed regulators were concerned whether users would understand they were being offered a choice of browsers and whether they would actually use the ballot screen.

In your view, is the proposed approach to downloading and installing competing browsers as seamless and straightforward as necessary from a user experience perspective for the ballot screen to be used effectively to exercise consumer choice between competing web browsers? the questionnaire asked.

It also asked respondents to comment on the proposed design and implementation of the ballot screen. The questionnaire dated from July, with a late August deadline set for replies. The Commission has not disclosed when it will decide on Microsoft's offer.

In an interview with the International Herald Tribune on Wednesday, Competition Commissioner Neelie Kroes said she wanted to reach a deal with Microsoft before her term ends this year.

Microsoft's Internet Explorer browser competes with those from Google, the Mozilla Foundation, Norwegian Opera, and others.

(Editing by Dan Lalor)

($1 = 0.6810 euro)