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.
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.
In the Arun district, 52 out of the 53 positive cases analysed were reported to have a non-Delta variant.
49 out of the 52 variations were the Omicron variant of the coronavirus.
This means that 94.2 per cent of the positive COVID-19 variations were of the Omicron variant.