Public Health England announced on Friday that the Delta variant of coronavirus is 60% more transmissible than the Alpha variant and now comprises over 90% of new cases in the UK.
The Delta strain was first found in India, while the Alpha variant was detected in Kent, southeast England.
Since last week, the number of Delta variant cases in the UK has surged by 243% to over 42,000.
According to data from the Office for National Statistics, the number of COVID-19 cases in England has reached a record high since April.
Public Health England also observed that Pfizer and AstraZeneca vaccines are 17% less effective against the Delta strain than the Alpha variant after one dose, but the difference in efficacy is minimal after two doses.
On Thursday, 7,393 new cases were recorded, with 44,008 people testing positive between June 4 and June 10. This increase shows a 63.2% rise over the previous one-week data.
As of June 9, over 40 million people have got their first dose of the vaccine, with another 28 million receiving their second shot. Vaccines are currently administered in two doses three weeks apart.
The R range for the UK has increased from 1.0 to 1.2 with the current growth rate rising from 0% to +3% per day. The R number is a mechanism for assessing a virus's ability to spread, with R denoting the number of people to whom a single infected individual may disseminate the infection.