In February, Microsoft and Yahoo announced that they were expanding their “search alliance” in the UK, France and Ireland. Today, they announced they’re about to enter the final stage of its implementation in these countries, and will start combining Bing and Yahoo search audiences on the Microsoft adCenter platform.

The ad transition officially begins on April 18.

“Next week, we begin to shift ad serving for Yahoo! Search over to Microsoft Advertising adCenter,” says Microsoft’s Cedric Chambraz. “This means that adCenter ads will gradually be displayed on Yahoo! Search result pages in the UK, France and Ireland. This process will begin as early as the 18th of April and is expected to conclude during the last week of the month when 100% of the Yahoo! traffic will be made available in adCenter and Yahoo! Search Marketing accounts will go in read-only mode.”

“As Yahoo! ad serving moves to adCenter, and your clicks and impressions in that account begin to increase, you’ll see a corresponding decrease in clicks and impressions in your Yahoo! Search Marketing account,” he adds. “It is thus important to continue managing your campaigns across both platforms throughout this transition phase, in order to avoid missing out on any potential clicks. To help you prepare, I wanted to share the following guidance to assist you through this transition.”

There’s a “transition portal” in the adCenter tab when you sign into your Yahoo account, which you can use to complete the transition, which the companies are urging you to do sooner rather than later, if you’re in one of these countries.

The companies also suggest increasing your adCenter budget (go figure) to prepare for increased traffic, once Yahoo search traffic starts to flood adCenter.

According to Microsoft, Germany, Austria and Switzerland will be getting the search alliance treatment in the coming months.