Yahoo is currently moving the back-end technology of Yahoo Search over to Microsoft's Bing platform and it will go live at various stages today. France, Germany, Italy and Spain are also making the switch from today.
The change first affects Yahoo-owned and operated sites, with Yahoo toolbars on partner sites making the switch by September.
Paid search will not be changed this year, according to a Yahoo spokeswoman.
In the US, Yahoo natural search first switched to Bing in October 2010, and the following month the paid search agreement took effect.
The 10-year agreement stipulates Microsoft takes a 12% share of revenue generated from paid search advertising on Yahoo.
Sales to high volume advertisers and search agencies are handled by Yahoo and sales to self-service advertisers by Microsoft's AdCenter.
The paid search agreement is currently only in effect in the US and the two companies are waiting until certain marketplace and product performance benchmarks are hit there before extending it to other countries.
However, Bing has replaced Yahoo search listings in eight markets excluding Europe and the US.
The two companies finalised the alliance in December 2009 and secured regulatory clearance in the US and Europe in February 2010.
At the time they set a timetable of early 2012 for having the alliance fully in place outside the US.
Google dwarfs both Yahoo and Bing in the UK search market, accounting for 88.7% of all searches, compared to Bing's 4% and Yahoo's 3.9%, according to ComScore data for June.
However, in the US search market ComScore's data for the same month shows Yahoo has a 17.5% share, Microsoft a 14.1% share and Google a 64.5% share.
Bing was officially launched in the UK in November 2009 after a five-month testing period.
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