The samples were checked by the Wellcome Sanger Institute in the week ending January 1, 2022.
Dr Hans Henri Kluge, the WHO’s regional director for Europe, said the highly contagious variant had become “a new west-to-east tidal wave sweeping across the region, on top of the Delta surge that all countries were managing until late 2021”.
He said: “It is quickly becoming the dominant virus in Western Europe and it is now spreading into the Balkans.
“At this rate, the Institute for Health Metrics and Evaluation forecasts that more than 50% of the population in the region will be infected with Omicron in the next six to eight weeks.”
But Dr Kluge said death rates “remain stable and continue to be highest in countries with high Covid-19 incidence combined with lower vaccination uptake”.
Omicron has now pushed Delta entirely out of many parts of England, research shows.
The Wellcome Sanger Institute analysed more than 34,000 positive Covid-19 tests taken in the week to January 1 to determine which variant they were.
In more than 30 local authority areas, Omicron was the only variant found.
In Wakefield, 329 samples were analysed, and 328 of those could determine which variant it was. In total 296 were Omicron (90.2%), and 32 were other variants.
The number of Delta samples found across England has fallen dramatically in recent weeks - suggesting that Omicron cases are replacing rather than adding to those caused by Delta.
But there are also pockets of the country where Delta remains a significant minority of cases.