Its chief scientist Soumya Swaminathan said on Friday: “The Delta variant is well on its way to becoming the dominant variant globally because of its increased transmissibility”.
The declaration came as a total of 75,953 confirmed and probable cases of the Covid-19 Delta variant had been identified in the UK, Public Health England (PHE) said – up by 33,630, or 79%, on the previous week.
Of the 75,953, 70,856 have been in England, 4,659 in Scotland, 254 in Northern Ireland and 184 in Wales.
The most recent data shows an overwhelming 99 per cent of confirmed and probable cases of coronavirus across the country are the Delta variant.
Meanwhile, a total of 806 people in England have been admitted to hospital with the Delta variant of Covid-19 as of June 14, a rise of 423 on the previous week, according to new PHE figures.
Of the 806, 527 (65 per cent) were unvaccinated, 135 (17 per cent) were more than 21 days after their first dose of vaccine, and 84 (10 per cent) were more than 14 days after their second dose.
As of June 14, there have been 73 deaths in England of people who were confirmed as having the Delta variant and who died within 28 days of a positive test.
Of this number, 34 (47 per cent) were unvaccinated, 10 (14 per cent) were more than 21 days after their first dose of vaccine and 26 (36 per cent) were more than 14 days after their second dose.
It comes as surge testing was rolled out in parts of Surrey amid a rise in cases of the Delta variant.
People living or working in specific parts of the county are being strongly encouraged to get a PCR test, whether or not they have symptoms.
The enhanced testing began on Friday and will run until Sunday for the areas of Banstead, Walton-on-the-Hill, Tattenham Corner and Tadworth, as well as Reigate town centre and some surrounding areas including Meadvale.
So far all confirmed cases have been instructed to self-isolate and their contacts have been identified, the Department of Health said.
Additional reporting by the Reuters news agency and the Press Association.