According to the Wellcome Sanger Institute, the figure in Sheffield amounted to 97.4 per cent of the 154 cases studied during the seven-day period.
Meanwhile, four other cases are found to be of other variants.
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 per cent 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 found in more than 30 local authority areas
Omicron has now pushed Delta entirely out of many parts of England, the research also shows.
The 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.
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.
According to studies, Omicron is less likely to make people seriously ill than previous Covid variants.
But newly mutated virus is still highly contagious and can infect people even if they are fully vaccinated.
The record number of people catching it has left health systems under severe strain.
In the UK, a total of 14,732,594 new cases were recorded as of January 11, an increase of 120,821 cases.
Government figures show a total of 19,828 people were in hospital in the UK with Covid-19 as of January 10, the highest number since February 16, 2021.