Google is warning Opera web browser users they must switch to Chrome in order to use Blogger.com, the search giant's blog-hosting service. No technical incompatibilities have been found to justify the alarm.

After a reader tip-off, El Reg created a Google blog using Opera 12.0 for Mac this morning, and received this message:

Blog off, Google

The site continues to operate just fine and dandy using Opera. However, the Blogger warning message is persistent: dismiss it, and it reappears. The alert box is banished if Opera changes its user-agent string - the line of text the browser sends to a server to identify itself - and pretends to be a build of Chrome.

An Opera spokesman told us: "We assume that this is an error and are talking to Google about fixing the the problem. With more than 270 million users on mobile products and desktop, we feel confident that it's in everyone's interest to support the Opera browser products."

It's not the first time an Opera rival has leveraged its own popular websites to steer users to its own browser. In 2003, MSN picked out visitors using browsers identified as Opera and sent those users a style sheet that shifted the web page 30 pixels to the left. When the Opera browser declared itself to be Netscape or Internet Explorer, the same page rendered perfectly.

Opera went on to detail incompatibilities on Microsoft's sites.

In 2007 Opera complained to the European Commission. In 2009, the commission ordered Microsoft to include a "browser ballot" screen on new Windows installations so that users can make an explicit choice of which browser to use. This was introduced in 2010.

Google is the subject of an antitrust investigation into alleged abuses of the advertising market: one complaint is that it favours its own websites and services higher in the rankings than they should be.

Britain's own trade regulator last year described Google as "a fantastic organisation that should be applauded".

Google had not responded to our request for comment by time of publication. We look forward to hearing its explanation. ®

Updated to add

A Google spokesman said: "For web applications to spring even farther ahead of traditional software, our teams need to make use of new capabilities available in modern browsers. So, like many of our Google apps, Blogger supports the latest versions of Firefox, Safari and Chrome, and the last two versions of Internet Explorer."