According to the latest update PHE said there were now 42,323 cases of the variant, which first originated in India, up from 12,431 in the week to June 3.
The variant is now dominant across the UK and thought to be 60 per cent more transmissible between household members than the Alpha variant, that started in Kent.
It accounts for more than 90 per cent of new Covid-19 cases in the UK with the rate of growth in infections across regions showing a doubling time of between 4.5 days to 11.5 days.
But PHE said it was “encouraging” to see that the rate of patients being admitted to hospital with Covid-19 was not yet showing a similar rate of growth. It said the roll out of vaccinations was helping to mitigate the impact among people with two doses, predominantly the older more at risk groups.
The Delta variant is better able to evade the vaccine immunity with one dose giving only a 33 per cent protection against infection. This rises to 80 per cent after a second dose.
In a new risk assessment PHE said: “Delta is predominant and all analyses find that it has a very substantial growth advantage. The observed high growth rate is likely to be due to a combination of transmissibility and immune escape.”
It added: “ There are now analyses from England and Scotland supporting a reduction in vaccine effectiveness for Delta compared to Alpha. This is more pronounced after one dose (absolute reduction in vaccine effectiveness against symptomatic infection of approximately 15 per cent to 20 per cent after one dose). Iterated analysis continues to show vaccine effectiveness against Delta is higher after two doses but that there is a reduction for Delta compared to Alpha. “
Public Health England are using new genetic testing that can deliver accurate results on the type of variant behind infection to help track the spread of the Delta variant around the UK. It is helping to target local actions to try and reduce the spread in areas such as Manchester, Bolton and Blackburn.
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. If you are eligible, we urge you to come forward and be vaccinated. Remember that 2 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.”
Yesterday Dr Harries warned the UK was going in the wrong direction in terms of Covid cases and warned there were “pockets” of unvaccinated older people who remained at risk from infection.
The latest Covid data shows there were 7,393 positive cases overall on Thursday, up 63 per cent in a week.