Microsoft has consolidated its entire $425m (£291m) global advertising account into McCann-Erickson, stripping the £7m UK account from Euro RSCG Wnek Gosper.
Euro has also lost the business in eight other markets including France, Germany, Singapore and China.
It is not known if the media planning and buying account, which is held in the UK by Media Planning, is also part of the review.
McCann, which already holds Microsoft’s $270m (£184m) US account, as well as the accounts in Japan, Thailand, Malaysia and Central America, is understood to have won £30m worth of business to add to its existing account.
Microsoft appointed Euro as its main UK agency in 1994. Last year the agency was forced to resign the £10m Ask Jeeves account after three months because of a clash with the Microsoft-owned search engine MSN.
At the time Chris Pinnington, managing director of Euro RSCG, said he felt the agency had to put its long-standing commitment to Microsoft first.
In October the computer giant launched a £150m global ad campaign for MSN through McCann.
The latest alignment is another success for McCann, which in 1999 snatched the US Microsoft advertising account from Wieden & Kennedy’s Portland office.
Last month Microsoft appointed Bartle Boggle Hegarty to handle the $75m (£51m) creative account for its forthcoming games console Xbox. A media agency will be appointed shortly.