Video report by ITV News Health Editor Emily Morgan
The Delta variant is 60% more transmissible than the Alpha (Kent) variant, new data from Public Health England (PHE) shows.
Cases of the Covid variant, which was first identified in India, are estimated to be doubling every four and a half days in parts of England.
The findings come just a day after Matt Hancock told MPs the Delta variant now represents 91% of new cases in the UK.
The question is no longer should they delay but rather how long for
The Covid variant, which was first identified in India, has taken over from the Alpha variant as the most dominant in the UK.
Since last week, the number of Delta variant cases across the UK has increased by 70% to 42,323, according to PHE.
The increase in the overall UK total – from 12,341 last week to 42,323 this week – has been partly driven by a reduction in test turnaround times and a faster process for identifying cases of the variant, PHE said.
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In England, 39,061 cases of the Delta variant of coronavirus have now been confirmed, along with 3,035 in Scotland, 184 in Wales and 43 in Northern Ireland.
As of June 7, there have been 42 deaths in England of people who were confirmed as having the Delta variant of Covid-19 and who died within 28 days of a positive test, according to PHE.
Of this number, 23 were unvaccinated, seven were more than 21 days after their first dose of vaccine and 12 were more than 14 days after their second dose.
Dr Jenny Harries, chief executive of the UK Health Security Agency, said: "With numbers of Delta variant cases on the rise across the country, vaccination is our best defence."
She said: "If you are eligible, we urge you to come forward and be vaccinated. Remember that two doses provide significantly more protection than a single dose.
"However, while vaccination reduces the risk of severe disease, it does not eliminate it.
"With data showing that Delta is significantly more transmissible than Alpha, it is just as important as ever to follow public health advice, which has not changed.
"Get vaccinated, work from home where you can and remember ‘hands, face, space, fresh air’ at all times. These measures work, and they save lives."
Meanwhile Lib Dem MP Layla Moran, chairwoman of the All-Party Parliamentary Group (APPG) on Coronavirus, said: "These figures should set alarm bells ringing in Government as we approach the June 21."
She added: "The Government must immediately explain to the public whether this exponential growth suggests the country is in line for a severe third wave, and if so what it is doing to prevent this."
The findings come just a week after Public Health England warned there is early evidence to suggest there is a "significantly increased risk of hospitalisation" from the Delta variant in comparison to the Alpha strain.
On Thursday, more than 7,000 new Covid cases were reported by the Department of Health for the second day in a row, according to the latest figures.
In total, 7,393 new cases have been reported in the UK in the last 24-hour reporting period.