The Delta (Indian) variant is 64% more transmissible than the Alpha (Kent) variant indoors and vaccines are less effective against it, Public Health England has said.
More than 90% of new COVID-19 cases in the UK are now the Delta variant.
The variant, first identified in India, has taken over from the Alpha variant as the most dominant in the UK.
Could the Delta variant delay the final step on PM’s roadmap?
Since last week, the number of Delta variant cases across the UK has increased by 243% to 42,323.
New PHE research suggests the Delta variant is associated with a 64% increased risk of household transmission compared with the Alpha variant – and is 40% more transmissible outdoors.
And Delta cases are doubling across all regions of the country in between 4.5 days to 11.5 days.
In England, 39,061 Delta variant cases have now been confirmed, with 2,035 in Scotland, 184 in Wales and 43 in Northern Ireland.
PHE said the large increase from 12,341 last week to 42,323 this week has been partly driven by faster test turnaround times and a quicker process for identifying cases of the variant.
The latest COVID-19 risk assessment by PHE reports the Pfizer and AstraZeneca vaccines are less effective against the Delta variant than the Alpha one by 17% after one dose.
But there was little reduction in efficacy after two, meaning getting a second dose is integral to protection against the Delta variant.
The PHE authors said: “This would support maximising vaccine uptake with two doses among vulnerable groups.”
As of 7 June, there have been 42 deaths in England of people confirmed as having the Delta variant and who died within 28 days of testing positive.
Of these people, 23 were unvaccinated, seven had had their first dose more than 21 days before and 12 had their second dose more than 14 days before.
The government is facing pressure to delay the lifting of most restrictions on 21 June but has so far said the data do not indicate the need to do so.
Nick Thomas-Symonds MP, Labour’s shadow home secretary, said: “These figures are terrible. The pace at which cases of the Delta variant continue to rise is deeply worrying and is putting the lifting of restrictions at risk.
“The blame for this lies with the prime Minister and his reckless refusal to act on Labour’s repeated warnings to secure our borders against COVID and its variants.”
Analysis: The Delta variant could push Freedom Day further down the line
By Ashish Joshi, health correspondent
We are so close to Freedom Day that any talk of delaying it will, naturally, come as a crushing blow.
On Monday, the prime minister is expected to tell the country if his 21 June target is still achievable.
Today’s numbers will inform that decision making. And the rising new infections driven by the Delta variant will give Boris Johnson some concern.
The variant first identified in India is more dangerous than any we have had to deal with so far.
It is much more transmissible than the original coronavirus and the Alpha variant first identified in Kent.
That is why we have seen such a significant jump in new cases in the past week. Infections will mean more people will fall sick.
We have seen a rise in hospitalisation in some areas in the north and north west. Not anywhere in the numbers seen during the first and second wave, but that does not mean it will not happen later.
That is why there are so many scientists calling for a delay to lockdown restrictions being lifted.
The argument from those who want to see a full lifting of all restrictions is that most of the “at risk” group has been vaccinated.
This is true. But there is still a significant number of adults who have not received both their jabs, which means people will fall sick and will need hospital care and it could lead to further delays in treatment for other people on the NHS waiting list.
It is true that the vaccines offer very good protection against the variant, especially after two doses, but it is not 100% protection.
The Delta variant is finding new people to infect every day. And that is with a record-breaking vaccination programme in full flow.
The light is still there but the tunnel might have just grown a little bit longer.