Time is nearly up for those web users devoted to Opera 12, Opera's last browser build to ship with Presto, as the Norwegian software firm will begin nudging users on to newer builds before mid-2014.

Given Opera's announced switch from its own Presto layout engine and custom JavaScript engine to WebKit and Chromium in February (and its subsequent move to Google's Blink ), it's little surprise Opera that would at some stage drop support for its legacy browser. However, until now, the company hasn't answered the question on exactly when that would happen.

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In July, when Opera released its first Blink/Chromium-based stable build, Opera 15, a developer on Opera's desktop team said it would deliver security updates for Opera 12 "for some time", in particular because the newer browsers were shipping without features that users liked in the older version. That, and the radical overhaul of the UI, meant it wasn't possible to force upgrades through an automatic update.

Opera confirmed version 12.15 for Linux, Mac and Windows would be updated to 12.16, but it wasn't clear whether it would be the next or last build — and it still hasn't been released.

Five months on from shipping its first new-generation browser and with the stable build of Opera 18 for Windows and Mac out yesterday, Krystian Kolondra, Opera Software's senior vice-president for desktop products, thinks the browser is nearly good enough to begin persuading those lingering on its Presto browser onto newer versions.

"We think we can offer people something to move to fully. In our builds that we have released so far, you can try experimental synchronisation, and already in the stable versions you can enable bookmarks, stars and things like that," Kolandra told ZDNet.

"There are some things that we are still working on getting and polishing and making sure this is a rock solid browser for everyone. So once we have that, we can recommend it really to everyone. Even people that have old browsing habits, even they could really go ahead and switch."

While die-hard fans of Opera 12 had hoped the company would support the browser for an extended time, Kolandra said it's probably going to pull the plug on the browser within the next six months.

"As of today, we're supporting Presto — whenever there is a security issue — and we still have a user base there and as long as this is big enough, we want to make sure these users can use our product in a safe manner," he said.

"But once we update everyone, of course we will be stopping. I can’t say whether it will be a one day and we stop immediately or we stop and wait some time for people to update. We will probably do the latter, but this moment will come eventually."

When asked for a specific date for the final release, Kolondra said: "We don't have a date yet, but definitely shorter than next year. It's likely to happen in the next six months."

At that point he said Opera would automatically upgrade Opera 12 users to a more recent version of its browser.

But will Opera be able force the move without losing significant chunk of its user base?

Kolondra thinks the numbers so far suggest they might stick around. "We're getting close to we will have 50 percent of people already on the new browser. We're really excited about that. Even without forcing uptake, we see uptake is quite encouraging."

Over the past year, Opera's desktop users have fallen from 55 million to 51 million, meaning it still has around 25 million users that it risks losing if they don't like the change.

But perhaps more importantly for the company, the value of those users in revenue terms has fallen even faster. Opera's desktop revenues in the third quarter of this year, driven by search dollars from Google under a two-year search deal set to expire in August 2014 , were just $13.8m.

That was down 25 percent on the $18m in desktop revenues it earned a year ago — when it was Opera's largest source of income. Now desktop is a distant third to mobile advertising and carrier dollars, which brought in $30m and $17m respectively that quarter.

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